I am thinking the 1.5" tubing is a bit small. Your engine is pretty good size now.
You could run the 1.5" tubing & step it up to a bigger size, that way it will still have great low end grunt & still be able to breath more for top end power.
Just a thought anyways.
Goodluck!
I am thinking my Clifford headers were 1 5/8 1.625 & worked great on my 254 CI L6. I'd have to go measure them to be sure or look @ a old Clifford catalog.
