Those heads on a 451 will run best at a higher rpm by a bunch. You might be losing a lot to bad valve springs and lousy valve control. A motor like yours shouldn't quit making power till well into the 7,000 rpm range. If you have very good rockers and pushrods, put some good springs on it and get a cam that has somewhere around 276 to 280 duration, 108 or slightly higher LSA, and .700+ lift. Isky has a RR735 grind that looks good on paper for your application. It has 280/288/110 duration numbers, .735/.684 lift numbers.
I have run this cam in my 528 and it pulled power past 7200 with a single four. If you cut your heads for some serious compression, (13+) this cam ought to make serious power. Put a 6,000 plus stall converter in front of it, and hang on.