You can move the rear back if you have the room , you just need to move back the springs , maybe there is a front hanger that will accomplish it or mount them inboard and just mount it further back . but make sure the rear isn't going to hit the gas tank.
I wouldn't use the poly bushings on the lower control arm ... 2 cents ...
What's the DRIVESHAFT PROBLEM ??? mount the trans , mount the rear , measure what you need and have it made , it's not rocket science .
What is the car that you found this engine in ? maybe its a 400 ??? same block height as the 383 , more cubes which isn't a bad thing .