Jamie, I could be wrong but I think a 37 or a 29 is the same footprint as the 20 long.
You would have a greater depth and you could aquascape upward.
As far as seeding the new tank if you don't want to mix in the crushed coral you could scoop some into a media bag and set it on the new substrate. Personally I put about 20-30 lbs of CC on top of my 120 lbs of aragamax in my 150. The sand bed critters and hermits mixed it in pretty well over time. I like a little chunk in my substrate. I've always felt it looks more natural.
I agree to using a 20 gal tote as a temporary measure and upgrade the tank or just get another 20 L. I've always liked that tank myself.
Good Luck.
Adam