I think you've heard from the others that these engines and drive trains are still quite usable even in today's environment. I'll address this issue from another side. First of all, these trucks are rapidly disappearing just from age and deterioration or they become compressed into a steel cube in a junk yard. Anything we can do to preserve them is welcomed.
You need to know your abilities if you are talking about major changes like swapping engines, drive trains or frames. These all take hugh amounts of time, effort and money. Many have started these types of projects and are yet to finish and ebay has many of these types of uncompleted projects for sale, which is an indicator of the time and money required.
Personally, my truck was converted to a V8 and automatic trans, before I got it, but the hatchet job that was done to accomplish this was horrible. I've yet to drive my truck because it wasn't running when I got it and will still take some more years and money before I get it in decent running shape.
I experienced what a delight these original trucks can be to ride in when Jim Shepard gave me a ride in his. These trucks can be quite wonderful without major modifications.
I would hope that you would work to get your truck running with it's original engine and drive train first. Enjoy it first, then move ot make changes to it as you go along.