i think any thing over 500 cid and you will want better heads you did not tell me the casting numers on your heads .the gm O ports heads are good for smaller builds BUT the ex port is not to hot. a set of 990 gm rect ports would work better there are may heads out there but the time you do your heads it would be better with a set the flow more . my old 572 i had to make headers for it and you would with you truck i think 2.1/4 to 2.3/8 tube is what you want , the prices on this stuff sounds in line