Have an S-10 of my own although it is ever so slightly newer in age, being a 2003, also an extended cab. Has the V-6 which I plan to keep as is; well sort of. Came to me with a true dual exhaust system that a previous owner had Rube Goldberg-ed into position. Great idea in theory, just very tight under there and must of cost more than had been budgeted for it cause the work started out nice and then got cheap. it's still there; I just re-imagined it and then created the infrastructure to match my concept. Biggest hassle was the O-2 sensors, major PITA to include them but necessary to keep the ECU's happy.
As for a possible engine swap, why not give some thought to an LS? Doesn't have to be an LSA or an LSX or one of the bigger cid versions. A 4.8 or 5.3 will do the job just nicely and both are strong enough as is. The biggest hassle will be the oil pan but Holley makes one with a shallower oil pick up sump that won't hang any lower than the crossmember. The rad has to be moved ahead and into its cradle but, there is room to do that. The truck model 5.3 that I personally scored came with the truck style manifold which sits taller than a car version. My idea to avoid having to modify the hood was to locate and purchase an LS6 manifold. They have a much lower profile and sit closer and deeper in the engine valley. And, of course, there is the engine management wiring harness to contend with. As has been noted and suggested elsewhere on the boaard, scoring the harness that matches the motor as part of the deal is one way of it, or going aftermarket and acquiring one that way is another.
On your fuel feed problems, Sitting that long, I would suggest that the original gasoline has long since turned into a sort of gooey varnish that has pretty much plugged up the filter sock on the bottom of the tank pickup. The cure for that is, you guessed it, to drop the tank and go for it. Not sure if your tank is steel or not, mine is heavy gauge PVC. Do know that you may have to shell out for new tank securing straps. Even in a dry and rust free environment you may still find they might break on being disassembled. It might be possible to toss in a can of Sea Foam Deep Creep gas conditioner but there are pros and cons to that exercise. It could dissolve the varnish and crud but that might also result in the filter sock dissolving as well; the varnish being all that is holding it together. If you do elect to pull the tank and inspect the sending unit/pickup, it would be easy and simple at that point to see if you can get the tank steamed out. Live steam ought to liquify and purge out all the residual crap that has accumulated from it just doing all that sitting. Just a thought.
Assuming the presence of a carb as opposed to TBI, there is also the possibility of an inline fuel filter in the carb bowl and again, the attendant possibility of a case of the galloping gas crud.
Just managed to access a couple of the thread additions and saw where you had back flushed the fuel line and got the system to move fuel cleanly again. Still think It might be worth the effort to drop the tank, or pull the bed and access the tank that way, depending on your patience level, and purge the tank out completely. It also lets you check the frame rails and give them a coat of anti-rust paint to protect them.
Nick