the 72-74 truck 8 3/4 housing is 59" outside to outside bearing cup.

same as 73-74 c-body 8 3/4 housing.

I have a 67 c-body Polara 8 3/4 housing with a 55.5" housing 46" perch center and a set of 30" axles. it may be close to the 59 1/4 over all width your measurements come up with.

I will throw a tape on a set of stock 8 3/4 truck axles in the housing for a over all width measurement.

I wager the 67 c-body housing is going to be real close. I came up with a 325/15/15 on a 10x15 with 3.5 bs when I did my measuring. BUT I did measure for the B-body housing and was very close to same measurements = I already have the 67 c-body housing,

if you look at pics of my tubs you can see I moved over to the floor/frame bracket. tubs flush with outside frame rails. did my stepside over to the floor/frame mount same way.

that would be best IMHO.