Do exactly what I did or you are a butthead
Here's the thing.
1- You are fortunate enough to have found a very low production all original unmolested truck that would be very easy to restore back to it's original condition.
2- What is the point of doing an LS swap on a truck you will most likely not be driving all over the place all the time? Does it need any more power than what was good enough all these years? And what about the rest of the old drivetrain handling all that new power it was never designed to handle when new? Would you be changing all that out then? Brings me to the question why would you need to start out with an original SWB 4wd when you could shorten a long frame from any year between 67 and 87 2wd or 4wd, or for that matter find a much more easy 73-87 SWB chassis that already has rear leaf springs. If you want a project, there ya go!
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WWSpKD? On my '72 K2500 with all original drivetrain and all original except a repaint in same color and a few add-ons I have owned since '01 I have gone through wanting to build a beefy stock looking 4wd with using 1t axles, either 4spd or TH400/NP205, 383 or Cummins, and lifted enough for the 37/12.50 Super Swamper SSRs mounted on white powder coated 16.5 x 8.25 wheels with hubcaps, and adding herringbone saddle buckets and console I had everything for, as well as the Sierra 3500 N.O.S. emblems. Time has passed and I have come to realize with it surviving this long remaining all original it would be foolish to deface it from what it originally was. I'm no purist, but I do have high regard for the merit of an all original survivors. There are others out there to play with. I have done reversible things since, that still follow the beefy stock theme with no permanent alterations. I will be building a 383 from another engine while storing the original safely.
I still want a big bad 1t 67-72 4wd to actually take off road and put through some real action, though still with care. Always was my style anyway.
I am buying a '77 K30 4spd/NP205 rolling chassis for $1k, putting the '70 cab light/high hump/AC cab on that I have, the front clip off my '72 once replaced with better, and building a bobbed flatbed for it. I am starting with parts saved from trucks that were not, creating one with a new life, built to my own personal satisfaction. This one my very well be my first LS-powered, if not Cummins or Turbo'd 292.
That's what I am doing. My serving of food for thought to you.