Best thing to do - as said above is start over with the wiring. Treat the Frankentruck like someone building a hot rod from scratch. You'll need specs and diagrams for both vehicles as a reference, but use one of these kits to get everything hooked up correctly.
/http://www.speedwaymotors.com/Speedway-Universal-20-Circuit-Wiring-Harness,42872.html
Haynes manuals are notoriously crap, but better than nothing. Good news is there are loads of Jeep fans online and I'm betting someone can point you in the right direction for diagrams and specs. The most important specs are going to be resistance values for things like your fuel sender and such. Adding resistors with the appropriate values is easy if you need to add ohms, it's much trickier if you need less. Summit Racing has lots of universal gauges and such for custom builds. I know it's nice to have the original ones work, but I don't know how realistic it is with the setup you have.
Good luck in whatever you decide to do!
