Drum to disc conversion questions

There were some small diameter, dual diaphram, boosters used from '67 to 69 that might work with your big block. The firewall bracket can be from any "A" body from 67 - 76. The factory clearanced the valve covers for this booster type and it was still a tight fit. If your going to have aftermarket valve covers you would have fitment issues even with the parts mentioned above.