I'm trying to determine the best header primary tube diameter for my smallblock. I have a moderate 350 running about 320 horsepower. I had the 1 3/4 primary's on my last headers. I'm wondering if I would benefit from the 1 5/8 primary tubes. I .have a flowmaster 2 1/2 inch exhaust system. I also have factory A/C in my 69 camaro so I also have clearance concerns. I'm not sure what the difference is in the 2 different sizes

You'd actually be better off going to a 1-1/2" primary tube about 40-42" long with a 2-1/4" collector.