400 HP is easy, 500 HP takes a little more knowledge and part selection. You really need to know what pistons and what deck hieght your motor has now before starting down the road to building more power I've built a bunch of 440 pump gas motors over the years, all the BB Mopars need and love more air and fuel in and out of the motor The more air with the proper ratio of fuel the more power the motor will make No magic here Real quick formula for 450 + hp, 9.5 to 1 compression on any pump swill, 850 CFM carb, 1 7/8 or 2.00 inch headers, Performer RPM intake, camshaft that has at least 240 degrees at .050 intake lift with as much lobe lift you can find( 1.6 rocker arm ratio is good ), ground on 106 to 110 lobe seperation angle, installed 4 to 6 degrees advance on the intake lobe center, heads that will flow at least 260 CFM at .500 and above valve opening. One of the last stock stroke pump gas motors I built with a set of midly ported 906 heads with 2.14 intake valves and1.81 ex. valves, Eddy Performer intake with a Holley 950 HP carb. made 535 HP at 6100 RPM and more torque than HP with a pretty big weird grind solid lifter cam,all on 91 octane CA pump swill The customer loves it, 1965 Satelite with all the extras, no light weight either Think twice, or more , act once