The rect headlight bucket is spot welded on from the factory but the mounting plane is the same. I converted a 74 to 80 back when, just used the chrome headlight door to locate, definitely no core support replacement necessary, just swap the bucket
I swapped a (low deck) 396 into a 74 shortbed, perhaps ignorant luck, but it dropped right in, had no issues but I had heard in the years since that the true big block frame brackets located the engine slightly forward perhaps. Had no problems with the headers either. The height difference is...
Sounds alot like timing too retarded...5-10 static, 36 total is a good starting point. Retarded timing requires more throttle opening, which causes the dieseling...
The whole vortec engine is better in the long run as it has factory roller cam. You will have to get a carb 8 bolt intake manifold to fit the Vortec heads, summit has them. This is a good swap
You must replace everything from the core support out, not an impossible job, just remember 4 headlight and two headlight groups are different. Ive done the swap in less than two hours, and no paint required
I would try to check the timing by advancing it to the point where it pings when you go drive it or fights the starter during cranking, then read it with a timing light and see what the pointer shows. It sounds like you may have a timing pointer or balancer inaccuracy and your timing is actually...