Have a shop that does performance engine builds do it. There is no budget way to make 1.5hp/cu inch. But you can do it less expensive. If the engine is a running core, you can re-use the spider and lifters. Have the heads done for performance. I'd have to snoop into cam profiles, and I'd run the MP single plane intake, and probably get some good forged dished pistons and run a cam tht wouldnt keep enough vacuum for power brakes but would bleed off pressure. The only issue is, the power wont be there until mid range rpm. Not a ton of torque below 3000 rpm. 425hp will cost you at least $5K done right by someone for you.