The rear end from the polara should be just right width wise, it's a little wider than the stocker by about 3/4" on each side, you'll probably have to weld new perches on or move the ones that are on it. As far as the engine swap is concerned it's all a matter of how much of the stock stuff you want to keep, the steering box will be the biggest obstacle it will want to occupy the same space as the driver side exhaust manifold, swapping to a rack and pinion will free up a lot of space in that area. On a big block engine the oil filter will want to be in the same space as the crossmember. Next will be the radiator and firewall, you can only move the radiator so far forward so the firewall will probably need to be massaged a little. there should be plenty of room in the tunnel as long as you don't mount the engine too high or too far back. None of these would be considered deal breakers in my opinion but it's going to take some fab work to put a big block in one of these cars. Disk brakes should be pretty high on the to do list if you go through with the swap.