To expound on Robert's points, the 392 shares the same external dimensions as the 345, so the swap is a piece of cake. This topic has been covered in great depth on several of the forums, but the quick 411 is as follows:
whenever swapping any IH sv from a vehicle other than a Scout II into a Scout II, some specific sii engine peripherals must be retained or acquired. The unique sii double sump oil pan, oil pump/pickup assy, oil dipstick tube, engine mounts and possibly portions of the throttle linkage assy. In your case, you'll for sure want to retain that delco dizz. Also, if the donor engine was originally mated to a manual trans, the flywheel must be replaced with a flexplate if it is going to be mated to an auto trans such as in your case. Since IH externally balanced their engines on an individual basis, swapping a ring gear from one to another May lead to engine vibration. You will also need to retain the spacer shim that goes between the starter and the trans on auto-equipped rigs. On the 392, you should find out if it is improved cooling or non-ic. The ic version is generally preferred, but that's not to suggest it should be the only deciding factor. Also, find out what carb and intake combos these various candidates have.
**addendum** you May also need to swap over the crank hub spacer and hub bolts depending on what the donor motor came out of and had for a trans. A lot of people forget about those...I almost did.