Allright. What kind of hell am I gonna go through trying this? ^^
So far, the major differences that I see are as follows:

On the wiring harness, the 81-85 has five sensors located just to the driver's side of the air filter, where the 79-80 has only the oil-thermo and the water temperature gauge sensors. The connector to the power lead is the same, with only a difference in missing pins (shown).



The 79-80 sensors and harness connector.


The 81-85 sensors and harness connector.

Has anyone tried this? Am I gonna have to refit the whole wiring harness to adjust for the difference in sensors?

 

Are you swapping the '79 engine into the '81-85 chassis, or vice versa? The 12A block is nearly identical, except the '79 will not have a threaded hole in the block for the oil pressure send. I don't think there's any other block differences.
If swapping the '79 block into an '81 - 85 chassis, just drill and tap some threads into the oil cooler line's banjo bolt for an oil pressure send. Use an '81 - 85 wiring harness, ignition system, carb and emissions.

...Or maybe I am misunderstanding the question?

 

It was the wiring in particular that I was concerned about.

It appears to be an 81-83 engine, which I'm putting into my '79 SA while we port and rebuild the '79 shortblock. Thanks for the advice.

 

I'm going thru the same problem have 1980 and installing 1983 rebuilt motor and trans it look like it might be beat to find a 83 or ECU and harness does anyone know what ECU might work?

 

Just swap the motor - if its 80 to 83, then use the 83 rats nest and such.

 

get rid of the nest. the only wiring u should be concerned about is wiring the dist of the 81-85 engine to the 79 chassis. the electronic one is better but if it's too much for ya, just put the dist from the 79 into the 81-85 engine.