Hello, Jayl13,
Blue Thunder, in my opinoin, has the right idea. Too big a port will kill power. I know i'm an idiot, but talk to the cylinder head guys at ANY well known manufacturer or head shop. On 454-496 engines, a properly done set up with the OVAL Merlins. You probably have SOME problem with your oval port set up. Flat tappet hydraulic engines set up like this with 454 inches should make 425++. Big port heads would lower the torque curve at least and not make any more top. Talk to a good engine builder and post, please!

I took my 1998 7.4 (310) hp motor apart and had the following. peanut port heads, roller cam and lifters, cast intake, stock exh. stock carb. 63 mph 22 scarab 23 prop. I swapped in, gm hi-po square port heads, crane "420" merc cam, forget the part # I needed to get new push rods due to the flat tappet cam, and valley pan. dart intake, stainless exh, new ign module without a rev limiter (be careful!!) edelbrock 750 carb and a HUGE gaffrig flame arrestor and royal purple oil. I picked up 11 mph on gps. I ran against the identicle boat with 435 dyno'd hp and were dead even. I figure the engine pulls 400ish hp and I run a 26 4 blade bravo 1. the prop is a little much, but I like the lower rpms. turn 4950 all day long. just be ready to throw away all of your old parts. or go with a roller cam, I didnt know it was a roller until after I bought all the stuff! selling the boat too 23 grand and its yours.
Billy

Loosecannon22,
If you use a different cam, but still hydraulic roller, with Merlin, GM aluminum, Edelbrock, or several others, all oval port, you would have much better bottom end and an even better top end!!!!