You can put any size of engine that you want in your '51 all the way up to a 454, but it's a stuff! Clearance for the steering box and the firewall are your main concerns. Even if you put a 350 (recommended) in, you have to make an indention for the distributor (no big deal, tho).
Check with the guys at Chassis Engineering for the best deal on mounts and further advice.
Enjoy.
