big bolt pattern for 7-1/4 rear??

Yeah, there were a few factory LBP 7-1/4's. I think most commonly found on 73 and later A body's with factory disk brakes and slant six or 318 engine car. Axles do not interchange as I think was already said. I think the LBP one's had c-clip axles too. I think they had larger brakes too.

Almost went that route myself when upagrading to front disks with LBP on my 73 Drum brake car. There are still 8-1/4's to be found though, I would pass on the effort to swap in another 7-1/4.