Ya absolutely need the small diameter booster to clear 440 valve covers if you ever expect to remove them. Might be able to shoe horn it past the larger one but I doubt it and will be almost zero clearance. Swapping the 3/4 ton dual diaphragm has been the ticket for years. Ya might also look at 70s C-bodies as they ran dual diaphragms too. They should all be 4 bolt masters and not sure about the firewall bolt pattern. As far as getting one remanned, last I knew the only place for that was carid and quality was suspect at best.