[moorepower]

If you are talking 289-351w idealy you would have a 1 5/8 header going into a 1 3/4 sidepipe collector. What you are doing is creating a set of step headers that are properly matched to your displacement. By using a larger header on this engine you will need to move the powerband up in RPM. This requires alot of camshaft, single plane race heads ect. The larger tube can be run but you will sacrifice power and torque below 3500-4000 rpm. Some af the headers I have seen come through the shop have been 1 5/8 primaries with a short 2" long section of 1 3/4 pipe to make it a slip fit on the 1 7/8 collector tubes. Unless you want to scrub off low end torque, use the 1 5/8 headers on a small block, i.e. 289-351W. moorepower