406's are fun (bored 400). I've had 3 of them. If you want more power though its going to mostly be achieved with the heads. You said stainless heads. From what company? Never heard of stainless heads. But worked vortec's make great power and the best SBC head is the AFR's. Switch to a hydrolic roller cam of the same size or bigger and go full roller rockers etc. The 415 in my racing partner's nova is a 415 (400 bored 60 with longer rods) Full roller valvetrain, .630 lift and 306 duration, Dart ported heads, Edl vic jr intake and a modified Holley 750 and was pushing like 503 crank HP. Add the 150 shot of gas and it was an easy 6.20 car in the 1/8th.