318 and 833 swap into 72 sedan

Whether or not you have to cut the input shaft is going to depend on the crankshaft in your new engine. If you have it out already have a look at the back end of the crankshaft and see if it has a deep bore to accept the input. Personally I wouldn't cut it unless you have to, and I would use one of the new style roller bearing pilot bearings that fit the large diameter (torque converter register) on the back of the crank. I have used them a couple times before, they are reasonably cheap from the Chrysler dealer and work great. You may have to lengthen a few wires on the harness but it will work fine.