M103 head interchange?

[Hit Man X]

I've heard yes and no, on being able to use early M103 heads on late motors, any truth to this?

Only thing I can see is the air injection bits are not on the early heads like on the late... and maybe the late cams didn't self EGR like the '89 and prior?

Reason being, I can pick up pre-EGR heads easily and for cheap. I need to rebuild the head on my '90 and would prefer to have limited down-time.