I picked the name project junkie 20 years ago, the last thing I need now is a bunch of stuff hanging over my head.... since you've bought yourself a 6.7 replacement and already have a centurion, I'd unload the 6.4 truck, possibly with the 12v if it was favorable to you, or hold it back for the Centurion.
I foresee stricter emissions issues in the future, like 50 state inspections ca type shit, I wouldn't want to dump a ton of time and money into building the perfect truck to last 20 years, and have it red tagged in 6 years by commies. You may have some leeway with putting a 96 12v into a 93 C350 but I think putting a non emissions engine in an 08 truck is betting on a future with less infringements
I'd clean it up, list it as a cummins candidate advertised in freer states, Idaho Montana Wyoming etc. I'd offer the engine as an additional option, at a higher than you think is fair price point, and if it doesn't sell, keep it for the big bronco.
Also consider a partial part out before selling it. If you've got $1500 worth of tuners and gauges on it, that can be pulled off, and it still runs the same, sell that shit separate.
If it's really a clean truck, the kind of truck that's worth swapping a cummins into, spend 3 hours washing and vacuuming it so it pops in the pictures.
Don't let the things you own, own you
Then find a fucked up house to remodel. That's what I'd do