Not to get knitpicky or anything, but a '51 GMC 1/2 ton would have had a 228 originally, not a 270. The 270 was only available in 450 series trucks in up.
Either way, the links above wrote the book on performance stuff for the 270 (or 228, 248, 302 etc) and they are the best in my opinion too.
TONS of upgrades you can do and you're only limit is your wallet!
Sky's the limit! Good luck and giddy up!
