1990 cab on 1986 frame?
 

Will a 1990 cab, & 300 e.f.i. motor convert to an '86 frame? The '86 has 300 carb motor which is bad. Would moving the cab from the '90, with the 300 efi help with wiring issues on the '86 frame. I have heard you would have to transfer the gas tanks from the '90 to the 86 frame, is this true?

The cab will go on the frame yes and would help with the wiring problem. You do have to switch the tanks if the 86 is carb simply because you will need the electric fuel pump to run the efi motor.

you may just need to switch the sending units as the newer one contains the pump so check them when all is apart ! you will need to make sure that there is a return line for the EFI !

A lot of work, but very doable. Goodluck!