Yes, however I found a few companies that make a dual 8 with a bracket, adapter, and a GM master made directly for 67-72.
What I am waiting for now is to see the actual OD of an 8 inch dual. My 8.75 inch stocker was very close to the Lemans V/C so if the dual 8 inch is slightly smaller, I will go that route instead of the dual 7.
I haven't been in a situation where the truck wouldn't stop, but it's a little too fast to be messing around and eventually I want to put discs on the front, so figure may as well replace the bad booster with something more