I recently picked up a 78 Aspen wagon /6 car. I'm planning to swap in an RB motor and auto trans. I'm wondering if there is an 8.75 housing that will bolt in there without too much modification?

Nothing is a direct bolt in. A 66-67 B body housing is the closest width, the 68-70 is a little wider but will also work. BOTH will need the spring pads to be removed and welded to the F body width.

68-70 B body is only 3/4" wider overall. You can move the springs easily. Move the front mount flush to the rail. You'll need to drill a clearance hole for the spring eye bolt. If you remove the rear bumper/bumper shocks the rear hanger can be moved inward and forward to the ideal location by simply drilling holes. use the B-body lower shock plate as well. The only part of the wheel well that's in the way is the outside front of the inner right behind the rear seat. A pie shaped piece as a filler works . The rear seat will still fit if you desire. Now a 10" wheel with a 10.5x29.5 will fit at normal ride height.
Doug

I have a build thread with a little bit of info on the BB engine swap here.

https://board.moparts.org/ubbthread.../76-volare-yes-a-volare.html#Post2116859

On the rear the 66/67 B body the spring locater pins are so close that you can "tweak" the springs in enough to set the rear without moving the spring mounts.

I ran a '66 housing in my M for a number of years. The first install I just pushed the springs outwards and kept the ISO rubber sandwich mounting for the rear. The only fabrication involved with this method was drilling out the spring pads on the axle with a 1 9/16" hole saw to accommodate the centering pin. I never liked this as you could actually see springs bowed outward from the rear of the car. Did this hurt anything? Probably not, ran the car like that for ten years.

Eventually I decided to do away with the ISO setup and mount the axle directly to the springs. Used B body shock plates and had normal B body spring perches welded to the axle further apart so I didn't have to kick the springs out. Changed the locating pins in the springs and if you plan on running a factory rear sway bar you'll need the mounting plates from Firm Feel. Those go between the axle and spring.

From what I understand there is no difference in the wheel wells from your 78 wagon vs my 87 sedan. With the '66 housing I was able to run a 27" tall 285 mounted on a 9" rim with 5" of backspace. It would rub a little on the inside of the wheel well in the front under hard turns.