I agree with HL about the 248 and 270 being the best choices.
The 302 would be the best upgrade, but be careful about getting a military version. Some military engines might need ome adaptations done to the block in order to work with "normal" bolt-on parts (fuel pump and generator, are two examples, I think). These dan be made to work well,
The 270 might be easier to find and was used for many years in domestic vehicles. 270s from the mid 50s to 1962 might be good choices.
The 248 is about half-way between a 235 and 261 Chev engine. It would probably be the best cost as a replacement.
