confused - or just a stupid question

One other thing. Was your cam installed "straight up" or was it advanced any? The power band of the 340 cam at just above Idle might not be able to break the tires loose. Did you try to race the Mercury Comet before and after the rebuild to compare?
One other thing, If your spinning, your gonna lose the race. It's better to grab some traction and go, then to sit and spin for a few seconds.

Your combination of cam, Intake, and ported heads probably breathe better up in the rpm band, above 2300rpm or more. add that to a low stall in your convertor, and highway gears. When brake-torqueing it trying to break the rear tires loose, your not up high enough in rpm's to do the job.

You may want to buy a line lock if your dead set about doing burn outs. A higher stall, more gear in the rearend, a line lock, all will help you to spin the tires.