trying to figure out which will be best with my build/budget.
Fresh rebuild 362 sbo
3a heads with bigger valves
425 bbo high comp pistons
comp cam dual energy 476-508 lift
RPM intake
Quadrajet stock carb.

This 1977 cutlass supreme will be solely a street car, but id like to get the most out of it while maintaining drivability. Anyone have experience with thornton manifolds or a set of long tube headers that fit well without extra modifications?

 

Hmmm kind of counter productive. You mention trying to get the most out if it while trying to retain drivability.
Headers will do nothing to adversely effect drivability. They'll only increase hp, tq and overall efficiency. If that's ok with you.

 

I have a 1976 olds, I put ceramic coated hedman short headers, I had no problems fitting them in. The only problem I had was on the drivers side. I had to make up and weld a cone shaped pipe at the bottom of the header, then run the pipe straight back to the mufflers. No issues running back off the drivers side.
Just an option I chose running 2 1/4 pipe. Our 76-77 year cars were not made to put long tube headers in without many issues.
I hope this helps,
Eric

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Correctly-sized long tube headers are worth 20+ HP over ANY manifolds. Carefully compare the Thornton SBO manifolds to stock ones and see if you think they are different enough to warrant the cost. Headers are essentially the same price.

 

What ever you do ditch the 3a heads.