The only real difference I see is the brake calipers. My current 27" frame will not accept a 700c wheel. Can I get longer brake calipers so the pads will reach the rim?

https://www.sheldonbrown.com/harris/brake-calipers.html

You can find a selection of long reach calipers here

If your frame doesn't accept a 700c wheel, changing the brakes is not going to help.

If it took a 27x1-1/4" tire, it should be good up to about 700x36C, tire clearance wise.

well the frame will accept the wheel, it's just the location of the brake that is piss poor.

Long-reach calipers will solve your problem. Keep in mind that your cornering clearance will suffer - if that's anything worth considering for you particularly. For some it may be an issue, for others, not. I believe Shimano has some long-reach calipers, but if you can't find those specifically, THESE should work just fine.

If you can't find any sidepulls you like, try ebay for some centerpulls. Weinmanns, Dia Compes, etc. are available regularly there for cheap, and there are models with extremely long reach if needed-

You might check this out. . .

https://www.sheldonbrown.com/home-drop.html

Like the drop bolt solution, but Harris Cyclery is sold out. Didn’t know anything like that exists. You can also use a round file to lower the slot for the brake pads (if there is enough material to do it safely). The difference between 27 and 700C is only 4mm. You will only need to remove half of that.

You're off a little. The difference in diameter between the two is 8mm (630 vs. 622). The dimension that determines the difference in brake reach is the difference in radius, which in this case is 4mm-

Long ago in a galaxy far away, drop bolts were pretty common and easy to find. Rumors abundant that 700 wheels were "the" reason the Europeans kept winning races, so the Americans were anxious to change over. (Ever notice how there's always just the ONE thing that's supposed to make the difference. . .? And it's never riding until you puke. . .)

If you can't find NOS (new old stock) by hunting around you might find some old ones at a bike recycling type of place. Short of that, you can tell by the photo on Sheldon's site that it isn't exactly the most difficult part in the world to make or have made if you've some stock and tap & die set.

Simplest/best/cheapest answer so far though has to be taking the ol' Dremel to the slots in the arms if you've got the space to do it.

I dunno about that, since race bikes in America came with 700c tubulars. The 27" vs 700c thing was strictly for clinchers-

You might have been more serious about racing or be a bit older than I am. . .

I noodled with the idea of going to tubulars and drop bolts would've been necessary and so on and so on. . .

Honestly, I've never ridden a tubular in my life. My first road bike was a '79 Schwinn Traveler I bought new in high school. But I had some friends back then who rode higher end bikes from the '70's (Raleigh Professional and Raleigh International to be precise), so I was exposed to them at least. But recently, a '72 Raleigh International has come into my possession, and so I've sought out quite a bit of information on tubulars. Bottom line, I'm building the Raleigh International up with some vintage 700c Mavic Module E clincher rims laced to the original Campagnolo hubs. These rims were not easy to find in 700c, they're some of the earliest hook edge rims for high pressure road clinchers that were available............Anyway, I didn't really mean anything by my remark in the earlier post, you just had me thinking of the vintage '72 tubular rims that were on my International when I got it, bought right here in the good ol' U.S.A. in 1972-

I got lucky, I guess. I am building lighter wheels for my Bridgestone RB-1, so I took the 700c wheels and handed them down to my Fuji S-12S, replacing the 27" original wheels. I was fully expecting to need to replace the brake calipers with something with longer reach, but the Dia Compe G sidepulls had plenty of adjustment to move the pads into position with some slot to spare.

You're off a little. The difference in diameter between the two is 8mm (630 vs. 622). Thanks for the catch. I knew it, but for some reason was having a brain fart.

Well-biked you're really, seriously testing my memory. . . your 700 clincher dates sound about right to me.

I've never ridden tubes either (and here in Albuquerque, probably couldn't -- nasty thorns called "goatheads" are everywhere!) but there was a period where I was seriously contemplating going to tubulars. A friend had 'em on a racing bike he bought used and was getting pretty serious, so was I, but less so. This would've been about 1973-ish (?).

It may be that Bicycling did a column on the "mechanical advantage" of the different wheel size or some such. . .? It could've been a juniors race written up in the USCF newsletter. . .? Anyhow, I was keen for any mechanical advantage, not being the strongest rider (at least, not as strong my buddy who had the "racing" bike).

Regardless, there was a time when drop bolts were pretty readily available -- in all the catalogs and back of the magazine ads -- but alas, that was the only part of the equation I was even close to being able to afford. The wheels themselves were hundreds of dollars even then. And thus ended my great racing career. (Add to which, I was using my bike as daily transportation to school and was more touring oriented and every hardware store in the country had 27's (back then)).

I'll still stand by my "one thing" theory however -- that folks want that "one thing" that will give them an edge and throw money at that particular idol despite the fact the motor on the bike is probably the most critical component, and riding until you vomit may just be a requisite. . .

I have some old Dia-Compe and Shimano brakes form an old 27" which seem to have a fairly long adjustment section ~ 10mm total

if its just 4mm difference won't most brakes still fit?

something to consider:
one thing i've noticed is that if the bike had bolt-on wheels (seems typical of old 27" wheels), a QR axle (seems typical of newer 700c wheels) is slightly larger in diameter.
I have a mid 70's nishiki international that had replacement 700c wheels and the axles didn't fit properly into the fork dropouts.
the dropouts where made just wide enough for a bolt on axle, so i had to sand down about 1 mm of each dropout to have it accept the 700c QR axles.
just my experience with it...