a 402 is a 396 bored 030 not really much difference, chevrolet produced them starting in 1970 and continued till 1975, they came in 375,350 and 325 horsepower in 70 and were the last of the high compression engines.Scince the engine is not the original it would be hard to say exactly what you have in there. You could get the casting and production numbers off the block, heads , intake ect and post them or go to <a href="http://www.mortec.com" target="_blank">www.mortec.com</a> and look them up yourself, if you post them here I'm sure someone will be able to tell you what you have there