the 78-88 A/G bodies, 80's S trucks, and 80's F bodies are the only applications for a solid axle that don't use the 4 bolt flange.
the BOP 8.5 axles with pressed on bearings in some 73-80something A and B bodies have the 4 bolt pattern, but it's different.
as far as bolting an 8.5 out of something like a 73-77 A body or 77-96 B body (they are pretty much the same chassis as each other..), it won't happen without a lot of work. while the mounts for the lower control arms, springs, and shock mounts can be cut off and transferred over, the bosses for the upper control arms are cast in totally different places and at different angles on the center section.
you used to be able to buy conversion upper arms for putting a 12 bolt out of a 64-72 A body, but i don't know if they are available any more.
so, to summarize- the cheapest and easiest way to put an 8.5 in a G body is to find one out of an 84-87 GN, T Type, or 442. it will cost more up front, but it bolts right in and you save a ton by not needing to do any fab work..