Keep the 460. It's cheap, (Gas cheaper than diesel) you can't beat the "howl" from it and the truck is set up for it: Literally KISS. Time to wring some power out of the 460 and there are many ways to do so for far less coin than a diesel swap will cost. Then you can spend some serious cash to get power out of a diesel.
DB2 oh, that's a pocket change $1200.00 injection pump. Followed by a $550 set of injectors. Followed by another diesel engine the DB2's failure took out completely. Yeah SCRAP METAL not a core... Anything happens with a diesel it's usually catastrophic and very expensive to recover from. Adding power to a diesel can better the odds to give you shrapnel impacted into the pavement aka hot oily scrap metal that used to be and may no longer resemble an engine. Sure your odds are better than ours with Ford IDI offering, but, surplus military engines give us a cost advantage...
So you got to get a diesel engine $, get all the adapters $, make it all play nice $time$, realize the transmission really isn't going to fing take diesel torque on what the 3rd $$rebuild$$, and when you are retired some point a shop working on Frankenstein is a problem for your limited income after your health won't let you turn your own wrenches. Fun things like all the hose clamps (16 on injectors alone) leaking on AZ heat dry rot hoses, swapping fuel filters, priming the fuel system after running out of fuel are high of the list of s#it retired people don't want to fk with anymore. Especially retired equipment fleet managers/mechanics that no longer own a specific hot rod 6.5TD burb.
Hope this clears the road ahead up for you a bit no matter what you decide.
Good used diesel trucks command the price they do because the above conversion is already done and is a known known.
@Will L. the new Cummins 4 banger crate engine is underpowered vs. a 460.
R2.8 Turbo Diesel
https://www.cummins.com/engines/repower