I have the same problem as you with my 88 Gulfstream HiRise with the F53 chassis and 460 engine, although my engine is an 87 and the last year for the carburetor engine. I do not have to pass an emissions test anymore so I took a little more serious approach. Since the subject is about Headers, I'll stay on subject. I used Flow Tech Headers (a division of Holley ) for a 70's - early 80's Ford F-150 4X4 and they fit perfect. My main concern was the parking brake linkage on the left side but it passes in between down tubes without a hitch. I had to use offset collector pipes (from Headman) to clear the transmission cross member and am using an "H" pipe along with Turbo mufflers. While Headers and a free flowing exhaust system will improve overall performance, the gains from that alone will not be Earth Shattering. Only a coordinated and somewhat expensive redesign will show major power gains.