Hey, I am in the process of putting a 350/th400 into my wifes 59 Rambler American wagon. Talk about a tight squeeze! I used a universal street rod crossmember for the motor mounts and mounted it between the control arm mounting brace. I mounted the engine just high enough that I could get spark plug wires on. I had to recessed the firewall back 4". I had to remove the heater and fill it in. You need to cut down the inner fender like farna said and weld in some panels. I put in a rack and pinion so I removed the steering box and made my own crossmember to mount the rack.
For exhaust you can use fenderwell headers like oddrodder , custom make some headers( I have some picts of some in a rambler american wagon) or like I used, a right side 265 chevy cast iron exhaust manifold and a 70 chevy driverside mainfold. I did this because these manifolds are rear dump out, there is no room for center dump ram horn exhaust or shorty headers.
A cross member is easy to make out of 2 x 3 rectangular I cut into the intragal frame (loosely named) and set the 2 x 3 inside and welded it up I also bolted it to the floor. I ran each side up to within 3 inches of the trans then made a drop out center sectionto hold the trans mount.
Rear end can be any one that fits in I think a rear out of a maverik
will fit. I used a extra rear out of a AMX and used the HD spring that were an option for the wagon.
I wanted to use what I had around and keep the cost down. If this is any way helpful your welcome. If you want any more info or picts let me know.