nhra rules..roll cage.firewall.fiberglass

You could use super stock cage regs as a guideline, but since that combination would be very expensive to run in SS, and since it sounds like you aren't going to be building a class legal car, then use the Super Street
(10.90) or Super Gas (9.90) rules as a guide. You are better off to overbuild the cage a little, especially if you think you might "get the itch" to go faster later.I think at the 10.90 level, a 10 point bar (not cage, some differences) is required. The rule book will make it all clear.
Talk to your chassis shop man and see what his input is. I would think your combination with a 3.91 gear is probably good for mid 11's, geared deeper it could run 10's. One of my buddies runs a similar 68 Barracuda with 11.5-1 compression and the 590/320 duration cam with 4.56 or 4.88 gears and a 9 inch converter.
While not a state of the art 10.90 class car, it was usually good for 10.95-11.05 et's. Had he fully tubbed it and updated it, it would be capable of better times.
The funny part was, his short block was originally found lying on it's side alongside a shed, with the pistons already in it! he rebuilt it, put ported worked heads on it, with the usual goodies.
Who says state of the art/high buck is the only way to get it done?!
Yay Mopar!
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Hope this helps, good luck with the project!
Alan627B