[LS short box]

I've used Painless kits on about 1/2 dozen hot rod projects. Most were LS swaps into something. Used the 21 curcuit universal. When I get the kit I lay the harness out and group the wires together. Back, front and under dash then tape together in groups. I'm less than good with electrical stuff but I've never had a problem.
My only gripe with Painless is that they supply really cheap butt connectors. I toss those in the trash. Then go to Fastenal and get the heat shrink connectors in bulk that have some type of goo inside that melts when you heat shrink them down. Never had a failure.
Not knowing your truck plans but I filled the stock junction block firewall hole. Then I moved the Painless fuse block back and fastened it to the panel next to the vent. Really cleans up the fire wall. Then I hid most of the wires going forward and back.