[120mm]

If you have a 302, the easiest way to get more power is to drop a 347 in there, or have the 302 stroked to 347.

A well set up 347 should give you 400 crank HP easy and still be driveable. It will also bolt right in without changing anything.

The next easiest is to go with a 351W or a stroker based on it, such as 408W. Running this requires new headers at a minimum.