Going back to the Wookiepedia article, and something I believe written on one of Decipher's SWCCG Y-Wing cards, the Y-Wings from Ep. IV were basically the Republic Y-Wing with most of the panels removed. I believe Decipher added the reason: they had so many mechanical problems the Rebel maintenance crews just left the panels off to shorten repair times.
When Joe Johnston refined the Colin Cantwell and Ralph McQuarrie Rebel Alliance Y-wing designs for construction into a visual effects model for Star Wars Episode IV: A New Hope, he posited that the starship was once a sleek vessel that was chopped and stripped like a hot rod by Rebel Alliance technicians. The BTL-B is apparently the stock version of the "stripped-down" Rebel ships.
So the Republic Y-Wing should have the same lines and size as the later Y-Wing. The biggest thing I'm seeing, as far as accuracy goes, is the nacelles being raised a bit from where they should be. But its a pretty good start, and it looks fun to play with. Nice job.