There are many years' GMT400 service manuals accessible as .pdfs, but unfortunately I don't have a link to them handy. Perhaps
@df2x4 will rescue me and post the link.
Measurements will tell you where to spend you time and money. Big-3 is neat but I'm not convinced it's a necessity in the general case.
I do think there's one area that should be addressed on every GMT400 (and many older GM vehicles): The headlight wiring.
The headlight circuit runs through the firewall into the cab, through the dash on/off and high/low switches, then back through the firewall into the engine bay and up to the radiator support. There is a lot of resistance in that path. A better, not necessarily best, simple remedy is to fly a couple relays in under the hood on the driver's side near the wheelwell, trigger their coils from the existing LtGrn and Tan wires from the headlight switch (gain access to them right there in the loom that goes along the driver's fender, cut them there), and ground the other side of their coils. Add a power supply lead from the fusebox stud to the relays, and connect the relays' outputs back to the existing LtGrn and Tan wire segments that lead to the headlights. This solution involves minimal wiring and garners most of the benefits of the fancier "harnesses" you'll hear about / find for sale, if you're using stock wattage incandescent bulbs. Higher wattage bulbs warrant other improvements and your DVM can tell you what needs attention.