Chrysler 8 1/4

Because the axles are made differently. The offset and backing plates are different. I know. It's stupid. But it won't work.

What does this mean?

"The only true bolt in is a 7.25 LBP unit."

A bodies came with 5 different rear ends. Dana 60, 8 3/4, 8 1/4 and the 7 1/4 big and small bolt pattern. So I don't understand the above statement.