mtn to road bike conversion -- top tube issues
 

I've got a really nice Sterling mtn frame that I think would make a great road bike. Trouble is, the top tube is really long. I'm most comfortable with a 52cm crank to seat tube height and a 54 cm top tube length w/ a 90 mm stem. The Sterling has a 57 cm top tube. I'm planning on putting drop bars on the bike. Can I get a super short stem? Or is there another way around this issue? I put Trekking bars and a shorter stem on my stumpjumper so I could use it as a touring bike which worked ok since the front of those bars shortened up the top tube by a few cm. But I'd really like to use something closer to drop bars on the Sterling frame. Any ideas or suggestions would be appreciated. thanks!

A stem swap shouldn't be a problem. Back in the quill stem days, I converted a Bridgestonhe MB3 to a commuter, and over the years I used stems from 150 down to 60mm (I still have half a dozen stems out in the garage). It changes the feel of the bike a little bit, but nothing you can't get used to in about 10 feet. I'd probably wait until I had the bars, mount them with the stem you have, then try to estimate what you need before you spend money on a stem.
You might consider height, too. There's no real reason in everyday riding to have the bars way lower than the saddle.

Why would it make a great road bike? It'll be using 26" wheels, the gearing will probably be too low and the geometry is different. Surely it would be easier just to get a road bike than effectively change half the components.

FWIW, I don't think you have to change anything, at least at first. Standard gearing on a road bike, 53-39 rings and a 26-tooth (or so) large cog, is too high for most people. Just look at the bikes you see on the road and check how many people are using the top four or five gears. Your MB may not be an ideal road frame, but that doesn't mean it won't work. I got back into cycling after a long post-college hiatus on a cheap mountain bike, then realized I was really more of a road rider. I did my first three centuries and hundreds of rides in the 40-60 mile range on that Mongoose with road tires and its original gearing before I could afford a real road bike, and when I built my Atlantis, I put on a 46-36-26 triple. I've never run out of gears.
Geometry also isn't likely to be a serious problem. My first mountain bike had 73-degree parallel angles, which also was common on touring bikes of the time. I can't remember the angles on the Atlantis (because in normal riding they don't matter), but I think they're 72-73. You'll be fine.

I use a 50/34 compact with a 12-25 block and I find quite often on fast group rides that I'm using 50/12. Most mountain bikes have a 44 as the largest chainring and I as spinning out frequently on my MTB with slicks. Considering I'm prone to spinning anyway I'd expect lower cadence riders to have even more of a problem.

I live in a really hilly area so I like the low 22t ring on a mtn bike. I just prefer a shorter top tube than is common on mtn bikes and am looking for ways to shorten up that distance a bit. I really like the long chain stays that are on the older mtb's b/c I like a touring geometry.

A dirt jumping stem should be short enough, they make them in the 45mm range I think. You won't get any rise from it though. This conversion may be more work than its worth. I went through this putting Dirt Drops on my Monocog 29er, and it is a great fit, but there are times when I'm not sure it was worth it. (every time I need parts nothing off the shelf will work, everything needs to be special ordered.)

I think most old school MTB's had 48 big rings and a 13 in the back. At least, all mine do. Cruising on the flats at 15-17mph is no problem, even when using North Road bars. My latest will have drops as soon as I can put together a suitable Craig's List Kit. The issue of shortening up the length is as easy as getting a short reach Nitto Technomic stem with an old school bar. With 1.25" tires, it'll feel just like a Trek 520 tour bike, only about 3lbs heavier. I know. I sold my Trek because it was redundant, and people are willing to give you what you paid for a "tour" bike, as opposed to a humble MTB conversion, which in my opinion is a stronger, more capable machine.

Maybe go with flipped Northroads or mustache bars? The Northroads would bring you back a bit compared to true drop bars.

I have a Giant MB that I am now riding mostly, on the streets. I used an adjustable stem riser to raise the handlebars, changed tires from 2.1" to 1.5" , & am pleased with it for MY uses. [ 5-10 miles per day ]. If I rode longer distances, etc,. I would have traded & gotten a " comfort " type bike, but this is OK for me.

Salsa makes a short-reach drop bar that'll help with the top tube issue. They call it "short n shallow."

If you wanna get progressively more radical, try Nitto Randonneurs, WTB Dirt Drops, or the On One Midge.