The rear mount locates the motor. All other mounts bow to the rear mount. It is the king. ^{1) 2) 3) 4)}
Always always ALWAYS install all the rear mount bolts first before tightening.

Attaching the rear mount to the motor:
All the bolts must pass through the mount with no binding or pulling. Shim the top 2 bolts if you have to.
Or file the bolt holes slightly to allow the mount to sit in place naturally.
Chase the threads with a tap if needed so you get no binding when turning a bolt in a hole by hand.
Snug the bolts down gradually to allow the mount to shift as needed as bolts are tightened.
Do not try to adjust the rear mount so the motor will fit in the frame. Adjust the frame instead (it is most likely the problem).

And tighten all of the rear mount bolts before installing any of the other mounts in the frame.
If you have trouble with one of the bolts, remove the others and start that one first.
Take your time to do it right, take the motor back out if you have to.