79 motor question
 

is a 12a from a 79 diffrent then a later 12a

There are some differences.

The '79 (and I think '80) engines don't have shutter valves, also the intake manifold on '79 (maybe '80 too) are different. I think I have heard the '79 manifold flows better. I think the carbs on the '79 are slightly different as far as location of the idle speed and mix screws. '79s also used a thermal reactor rather than exhaust manifold and a heat exchanger instead of catalytic converters. And trannys are different too, but thats not really the engine. Thats all I know of.

Rotors and the assembles are different weights, Heavier I believe. I have an 80 engine in my 85 GSL rigth now, while I think about getting started on the rebuild/porting of the 85 engine.