parts are easy to find for the '54. www.chevsofthe40s.com is just 1 of the suppliers.

350 will fit. chevsofthe40s has the mounts and things. you'll have to cut the ribs
out of the firewall, that are on each side of the resess ( for the 6 )

Look for a solid car, maby even a 4dr, as that will be cheaper ( you want heavy mods, so a 2dr conversion is possible )

as for a top chop. don't have the roof cut, w/o having someone agree they will cut the rear window down 4 you. there not reproduced that I know of, and finding someone to cut old glass is going to be hard. '54 Chevy has got to be 1 of the hardest chops, so it's gonna cost you $$$ and being your just starting, you don't have the ability to chop it yourself