When I first got my 66, I went with Casler long tubes. They fit perfectly, of course I don't have PS. I got tired of tightening the header flange bolts all the time, and the heat radiating from under the car, etcetera, so I went to the Ford dealer and bought HP manifolds. Never been sorry.Shorties or cast HiPo's... which will both perform about the same....except the HiPo's won't leak, blow gaskets, loosen up, or add unneeded heat under the hood.
Funny thing, the Caslers fit perfectly, and the original Tri-Y's on Shelbys fit perfectly (I've worked with a few) back when this stuff was made practically by hand on fixtures. Nowadays, with all the whiz-bang computer technology, it's hard to find headers that don't need to be worked over with a hammer to fit. Strange.