I can help you out. You really only want the big collector size on race motors that rev high and aren't expected to make good torque down low. If you put those 3" collectors on your truck's 283, you will lose torque, and trucks need torque.
I would also like to recommend an X-pipe crossover, because the superior scavenging will pull more torque out of your motor, probably as much as 10 more lb-ft. After that, if you want to open it back up for your stacks, no problem.
Actually, if you can get a Tri-Y header from Doug Thorleys Headers, you'll realize even more torque.
But if not, then a full length 4 into 1 header style is the next best. Shorty headers are better for upper rpm horsepower, but full length headers make better torque.
