Mental help needed, please.

[Kalums]

Ok, I'm having straight forward decision making "funk" for lack of a better term. I need you all to help. Not so short back story: I've got a 1971 GMC K2500/flatbed, with a 350/ TH350/ NP205, Dana 70 dually rear-end, Dana 44 front 4.10 gears( Un-confirmed, that is what the "tag" on both axles says). I've already done a complete tune up(plugs, wires, cap, rotor, new HEI innerds, including weights and springs) rebuilt the Edelbrock 1406 that came with it and completely rebuilt the fuel system(tank, sending unit, fuel filter, fuel pump, fuel line to the carb) I've also replaced the starter and flex plate (both were missing teeth).I've also replaced mounts/lights/ and other aesthetic parts. .. At this point I'm getting frustrated... It will not time correctly or run smooth. I've tried finding TDC using a piston stop and it is way off(yes I rotated it by hand both directions and then found the middle) it will not stay "timed" I set it 12 dgrs advanced, shut of the truck, come back 10 mins later and it is "out" by 2 dgrs(either direction). There is also noticeable slop when turning the crank by hand to when the rotor actually starts turning. This to me tells me my chain/gears are toast and at some point I've developed a tick/tap in the driver's side of the engine that sound suspiciously like a lifter/valve/rocker or similar on #7.
**At this point I'm at a cross-roads.**
1) Do I continue to patch this possibly original ( I think the heads were replaced at some point) 53 year old engine "because it runs"?
OR
2) Swap in a Factory GM long block "TBI" 350 crate motor that has been sitting in the crate since 2017?

Any and all advice is welcome/appreciated.