Stock exhaust manifolds vs headers

Since the slant six is "two holes down" it's a much more systematic approach than a V8. INstead of putting on one part at a time and seeing results, the slant six needs all the systems upgraded to see good results. I had the stock manifold on mine ported and the outlet opened up to 2.5" and ran 2.5" pipe all the way back. It made a nice difference. Mine is also over 10:1 compression, has a 450 Quick Fuel four barrel and a big solid lifter cam.