I think I know the answer to this, but I want to confirm before I make a purchase - are engines interchangable between years? I know there are displacement differences, compression differences, etc. I'm wondering about fitting a later engine into an earlier car.

Specifically, will a GC code from a '75 fit in my '74?

Yes and yes.

Not much difference, GC often have smog holes in the heads that need to be plugged, '75-76 exhaust sucks so backdate with -'74.

Any used runner /4 engine check the exhaust studs for damage.

Thanks for the info. I'll be using the exhaust setup that came off my thrashed '74 engine.

Are the exhaust studs difficult to replace? What do the smog holes need to be plugged with?

The exhaust might not fit. I think it's 1.8 and 2.0 that r the same. 1.7 different. I know someone more knowledgable will confirm or correct.

If the injector tubes are still in the head, you can use a 3/8" NPT pipe cap.

I forget the thread pitch on them, but a bolt can be screwed into them. Remember, they go into the exhaust port so you don't want it sticking into the stream very far.

Last method is to have them welded from the inside.

1.7 / 1.8 are the same. 2.0 differes. Use the heat exchangers with correct muffler and either one will work. They must be used as a pair.

I concur. The 2.0 heat exchangers are straight, whereas the others curve up a little. Therefore, you have to use 2.0 heat exchangers with a 2.0 exhaust. I have a few sets of 2.0 heat exchangers if you need them. I don't think I have a 2.0 muffler.