67-69 Dart swap questions

It's got the 318 in it now, so that helps the pocketbook some. I don't exactly intend to be all that kind to the rear end though, what's the fun of a Mopar unless you drive it like you mean it =P? I figured the 8 3/4 was inevitable, just didn't know if it would last long as is. As it stands the car should run and drive, so I intend to sort of do things as weekend projects until I find a hemi I can drop in it. I assume I need to start looking if I want to find an 8 3/4? We had a 69 Charger a while back that desperately needed one to back up the 440 we built for it, but they were mighty hard to find and awfully expensive for anything with a posi in it. The 4 speed is in it to stay, so what do I need to consider for getting it to fit a new engine? I thought someone said that you had to shorten the input shaft (at least with the Mopar performance clutch and flywheel setup) which is not problem, but do I need to worry about any mounting changes or can I pretty much just pull the old engine and drop the new one in?