61-64 Chevy frames are not even close to a Pontiac frame. If you intend to tie into the frame, not the floor. I doubt that the chevy X frame kit is going to have the correct tie in points as on a Pontiac swept hip frame.
I.E. the Pontiac frame rails run just under the rocker panels similar to the A bodies, the chevy frames run as a common backbone through the center of the frame forming an X that the 2-piece driveshaft with a carrier bearing that runs through the X portion of the frame, much further towards the center of the car.
If you intend on bolting the hoop to the floor, it could work. In my thinking a roll bar hoop tied into the floorpan is just a visual way to satisfy the rule requirement. In a severe rollover wreck, it's going to be inferior to protecting the driver. In an oval track car, the roll bar uprights must be welded to the frame for the driver's protection. A rollbar not tied into the frame is about like using exhaust pipe in place of rollbar tubing, it's going to bend and deform when you need it the most.
In my racecars I always wanted a 4-post cage because in a rollover the A pillars usually collapse first and the roof ends up on the dashboard. Not much room leftover for your upper body when the roof collapses in the front.
