I'm planning on getting a 29er (dawes deadeye) off bikesdirect and I'm unclear on how sizing might be different - especially if I can't test the bike in person. I'm 6'1, 35" inseam, and as far as I know my arms aren't oddly long or short for my height. I rode my dad's 21" Trek 850 Antelope and it felt just right but it has 26" wheels and a horizontal top tube. I also know my 18" Raleigh Mojave is too small. I'm just trying to pick between 19" and 21" now. Any help would be much appreciated

19".

You are on the fence. Either size will probably work. Is your 18" way too small for you or barely too small for you?

18" is pretty small. I went in to my LBS today to test out a few for size. I rode a 20.5" Norco Judan and it was very comfortable. The standover clearance was a little small. If I were buying one specifically for anything technical or risky I would drop to the 19" for more clearance. Since this will be more of a winter commuter beater/mild single track bike it's probably better to go with the 21" since it's more comfortable for leg and arm extension. Also, the Judan 21" (20.5 wasn't listed on their site. the must've rounded up) had a 33.4" standover height and the Deadeye 21" has a 32.8" standover.

I would try to match the top tube length with a bike that is a known good fit for you. TT is probably more critical than ST length in terms of the right fit.

Your Schwinn is pretty PIMP. I have a '78 SuperLeTour 12.2 that I similarly pilaged.

I'd say stick with a 19". If you want to be a little mor stretched out but have some crotch clearance, snag a slightly longer stem and set back post. You would probably feel pretty good on it stock if you just give it a couple wks to get used to though.