I'd be inclined to suggest a reman engine such as those offered by Jasper, or GM's Target Master.
Having yours rebuilt will take time, while the reman is basically a straight swap that can be done in a day easily.
If you have the space/tools/ability etc to do your own rebuild, and don't mind the down time, it can be a satisfying project. But if it's a work truck, I'd say "reman".
'75 GMC "Blood, Sweat & Gear$"