[dsraven]

there is also the tried and true small block option. prolly easy enough to find mounts, manifolds, bellhousing, etc etc.
up to you. I just don't see the economics in spending money on an engine that is not common. I would, personally, cut my losses and swap somethghing common in there. thats me though. it is your truck and your dream so you need to build it like you want it. no looking back.
I suggest to look at all the things that would need to be done.
fuel system, do you wanna keep a fuel tank in the cab, right behind you? fumes, leakage, etc.
exhaust, is the current system viable or will it need to be replaced anyway
wiring, are you good at wiring to be passable should you wanna swap in a sbc or an LS engine
trans/clutch, is the current clutch and trans good to go with the different "old" engine or will that need some work
rad, will the current rad work with the different "old" engine
engine mounts, will the current engine mounts work with the different "old" engine or do they need an upgrade
bellhousing and rear mounts, will the current stuuf work with the different "old" engine or will there need to be cash spent
gauges, what will work
linkages

I guess what I'm saying is do a complete feasability study on doing the upgrade to swap in the 'old" engine compared to upgrading to something modern, or more modern. do the math and decide. a few hundred for this, a few hundred for that, new hoses, flush rad, new clutch, pretty soon you have a good sized bill to install an old partially worn out engine that may work out well or may start burning oil and leaking oil shortly after you start using it. not trying to be a downer here, just trying to give insight and help you make an informed decision.