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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.

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I think we are on the same page here Greg. Big cam with small cubes is going to need more rpm. I think it has valvetrain instability, which will weaken the springs prematurely. Could be a combination of both at this point. I do think a smaller duration cam would help the bottom end some and avoid more gear and maybe get away with his current converter to get that 9 sec slip. It all comes back to whether or not he can turn the needed rpm without the motor laying down.

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First off let me say Thanks for everyone input

Now new springs are n the upgrade list but just trying to figure out this cam deal, so I can get the right ones. the conv is out and will be sending it to UCC for a refreshing and talk to Lenny about spec change. PLEASE don't beat me to bad on this one but rocker are crane gold 1.5s