There are sooo many 70-80's 27" road bikes in the Trash Garage sales and Discount stores and many of them have good lugged frames and decent components.

They are perfect canidates for flatbar townie bikes and commuter with one exception.
Poor tire selection.
Tires wider than 27 1-1/4 are very hard to find. I.E. racing tires only
I have found some 27 - 1-3/8 street tires recently for a bike I am building for my 12 year old daughter from a 52 mm lugged lightweight frame with the 27" wheels. I bought 2 sets

Take a look at your 105 brakes as they are currently set up for your 27" rims. If you can unbolt the brake pads and they can drop 4mm in their slots then it's possible. Do this for both front and rear. Now, borrow a friend's set of 700c wheels and see if you can make it happen setting the pads up to hit actual rims. If you're almost there but not quite it's possible to extend the slot on the caliper arms to allow the pads to drop lower using a rattail file or a dremel tool. Don't go crazy here, use common sense.

[clueless] i would assume that 700c wheels have a smaller diameter? since you're moving the brake calipers down from the 27"?
i had always thought that 700c had a bigger diameter.... or are they the same? [/clueless]

"700c" wheels are 622 mm; "27 inch" wheels are 630 mm.

For details, see my article on Tire Sizing:

https://sheldonbrown.com/tire-sizing

Sheldon ISO/E.T.R.T.O." Brown

so, according to those charts and what sheldon brown just posted, 27" actually have a larger diameter than 700c?

i suppose the confusing part was the name 29" given to 700c
too much to think about, i tell you!

So is there an easy way to convert these old bikes with a new brake caliper or drop bolt that would allow enough clearance for the slightly smaller rim size of a 700c. Or even a 26" set-up. Then that would open up the possibilities for a much better selection of tires and wheels.

Hi my name is Fred. I'm running 27 x 1 1/4's w/ Prestas. I just get what's in stock at the LBS. Selection? Fooey!

Yes, most of the brakes on these older bikes seem to have the extra 4mm reach for 700c. I Just dont want to buy new wheels when these are perfectly good.
But you are right. The selection is much greater with the Cyclecross type treads in 700c

Here are a few pics of a the latest work in progress. The tires are 27X 1-3/8 and I like them better than the skinnys for commuting around town and on the packed limestone trails

My LBS sells 27 inch tires. Some don't. There isn't a huge selection of 27 inch tires, but generally the ones that exist are the ones most people want. Only thing really missing in 27 inch that I might like is studded tires.