Your Cleveland exhaust system affects performance as much as any other part of the engine. The stock exhaust manifolds are too restrictive. With restriction comes excessive backpressure and the contamination of fresh air and fuel causing poor performance
Header selection: With the Cleveland stock manifolds versus the long-tuber headers you can expect between 20 and 30 horsepower and comparable torque. Headers allows the hot gasses to pass through very quickly versus the manifolds.
The first thing you want to do is figure out what the proper diameter you need from flange to tailpipe. The primary tube size and length are determined by where you want your torque curve. So basically you need to decide where you want your peak torque. The smaller primary tubes gives better low to mid range torque. *Do not to go small or it will have the opposite affect and cause you to lose horse power and torque.
Calculating Primary Tube Size: This is a very advance and complex process. Everyone I know buys headers straight from the shelves or catalogs. There are just so many variables that you must consider. Heads, induction, cam, and compression. The question you must ask yourself first is what you want your engine to do? The bigger sizes are for non-street cars. The street cars you want torque because that is what you use most of the time.
The ceramic =coated hooker long tube headers (PN 6211 – 1HKR)
for the Cleveland are designed to accomplish two things torque and horsepower. The Primary tubes are 2 inches by 34 inches with a 3 1/2-inch collector.
Headers exist to reduce exhaust restriction and improve scavenging. You want exhaust to exit the combustion chamber more aggressively yet hold enough back pressure to help torque. You have two things to consider primary pipe diameter and the length right off the header to the head. Both determine how horsepower and torque are made.
It has been proven time and time again that shorty headers are not nearly as beneficial as long tube headers.
I highly recommend buying the book by George Reid, “Ford 351 Cleveland Engines”