The "J" shift can be a bit tricky if you want to relocate your shifter or use cables.
The challenge is the the slotted interface on the shift-rails need a 'pin' at 90* in the neutral position to operate them reliably.
the first step would be to change the interface by ether swapping in replacement "T" type rails (we carry them), or to cross drill the shift-rails 1/4" to use a spherical rod end linkage ("T" type shifter).
The cable interface we use allows the cables to come in from the rear. Shown here with the optional interlock bar.