Okay, so what you have there is an old 3906 transmission and some metal ( aluminium maybe- are they magnetic?, heavy or light feeling for their size? ) output gears- those are no good for your G4 truck.
You need to hit up ebay for a complete Emaxx 3903/05/08 transmission and motor mount + slipper clutch ( though the slipper can be upgraded easily enough if you need to, there are a couple good options about ).
You can always stick the 3906 tranny back in the old emaxx and run that as it is or let friends drive it etc, to spare your nice truck from their bad driving
