i just recently had to replace my old Edelbrock headers with a set of flowtech (moneys tight so i went with cheep but alright)
anyways
the new headers have Oval Exhaust ports, where as my block has Rectangular ports. meaning there is a small (very small) overlap between the headers and the ports. each corner has a little place where the headers are smaller than the block ports.
How badly would this effect performance?? and is there anything else i would have to worry about with this?

 

90% of guys running headers on older cars are running round tubes on square port heads. optimal? no. hp loss? minor. The flowtech headers are only 1.5" tubes, so the port mismatch is probably the least of your concerns. That's on a 350?

 

Yeah its on my 350 (from an 88gm truck)
but turns out the flowtech headers stuck out a little to far anyways so they'd rub the frame. So i just went with a set of Hooker Shortys with a Y-pipe