The guys that really know those old GMC 6 cylinders are on
http://www.oldgmctrucks.com/. Click on the "Discussion Boards". You have to log in.
There are some guys there that have hopped up the 302.
Obscene horsepower by '50s standards, maybe. High performance parts are only going to be found on forum message boards, ebay or custom machined.
If you want a high performance inline six, you can get a lot more power for a lot less money with a 292. There are many more sources of information and parts. The 67-72 trucks came with the 292 so you have a lot more to start with.
If you were building an older truck from the early '50s, '40s or '30s then the 302 would be a worthy addition and even period correct.