with that much torque you should be able to roast the tires from even a 20mph role.... I would do a compression test.... I'll bet the motor is a little tired.... Or is in dire need of a good tune to make it work...

Here is a recipe straight out of the Engine Masters spring 2005 book....

454 big block with forged internals 8.6:1 comp rated 425hp
472 actual hp 500

Edelbrock RPM intake

Test #1 cam XE268H Peak hp 483 @ 5300
Peak tq. 541 @ 3900
Average TQ. 506 3000-5600
Idle vac 15.7 cranking comp 165

this is with 118cc rectangle port heads

test #2 XE274H Peak TQ 538 @ 4000
Peak HP 508 @ 5600
peaks are up +5 and idle vac drops to 12.4"

these were done with a 850 demon carb.....