[25stevem]

Stroking the motor with those stock heads and a street cam to better feed the added CID will only pick you up like 30 hp , but you will have a lot more torque and the full peak hp will come in at a much lower rpm then stock.

Also if your not going to run headers then once again with stock heads I can see you gaining more then 30 some hp like I posted.

This is because a 421 motor already has the stock heads tapped out by 4800 rpm at best!

I can also guarantee you that the stock 376 hp rating of that motor was exaggerated!

Pontiac engine builder Jim Taylor a number of years ago built a stock stroke 421 with a 66 tripower sized carbs, ported heads flowing 218 cfm up from the stock mid 180s a long with a 11 to 1 compression and a big cam to tame that cylinder pressure down some and produced 475 hp thru what I recall to be headers.

One first thing you need to do is think about a ball park range as to how much hp you want the build to make!