From Wikipedia:
The 302 GMC inline six was produced from 1952 to 1960, when it was replaced by the V6. It has a 4.00" bore and 4.00" stroke. Connecting rod length was 7.000". This is the largest raised-deck engine. It was originally designed for the GMC military M135 and M211. It was used in military 2.5 ton trucks with the HydraMatic transmission, however the engine was a sealed engine for snorkel/submersion use, had an electric fuel pump, and other features such as a deep sump oil pan. From 1952-1959, GMC manufactured the civilian 302 engine which was not sealed, had a mechanical fuel pump, and used a "standard" oil pan. This engine is popular with hotrod enthusiasts because it delivers tremendous power for an inline six engine, is truck built with a heavy cast block, and can take quite a bit of abuse.
At one time there were a lot of aftermarket speed parts (intakes, headers, camshafts, etc.) for this engine.
I would try to keep it original unless you are not concerned with having civy parts on it. There should be plenty of stock parts available thru the surplus outlets if you can't find on via SS.