[donut]

If you have a tbh350 in the truck now, the easiest would be finding a 4x4 tbh400 and transfer case (and more than likely driveshafts) out of a junkyard.

The tbh350 is a 27 spline output, the tbh400 is 32 spline. AFAIK the existing transfer case won't work (in it's current form). The case can be reworked to fit to the tbh400, but you still need to get the adapter. Our 4x4's don't use long or short shaft like the 2wd, but go directly to an adapter then coupled to the transfer case.

I mention driveshafts because I believe that the tbh400 is physically longer than the tbh350.

I'm not an expert, I'm sure that someone with a ton more knowledge will chime in shortly.
At least I can give you some things to think about. I know it's not a direct swap, otherwise my old k-20 would have had the tbh400, instead of me having the tbh350 rebuilt. (I had both in the barn.)

The 454 will bolt right up to the tbh350, how long it will last is unknown.