That's a decent price. I'd say it's pretty much in line with what 531 Moto frames go for on eBay.
~2 years ago I picked up a '78GJ complete for $80 but by then they had switched to Vitus 888 tubing and SunTour/Sugino drivetrain. The 888 ain't bad but Reynolds 531 is better. The ST stuff, to be honest, works better than the Huret or even Campy I've seen on some GJs but doesn't have the "panache."
FWIW my bike took 700C with the stock Weinman sidepulls. The rear just fit but it did indeed. I don't know if the specs carried over from the 531 frames or not but I'd say you at least got a chance. Assuming you have a 700C wheel sitting around it'll just take a minute or 2 to find out. If it doesn't, I'd agree with Kurt and stay at 27".
Good luck on the build.
