Any relay will do it, it can be as fancy or as simple as you want. As far as the wiring is concerned th P.O. has already shown that their wiring skills were limited, there is a chance that the 4th gear "lock in/out" switch is there but not wired in and this is a risk, as having the o.d. in reverse will destroy it. That being said 8 forward gears sounds fun (but is probaly more torque than th eo.d. was made for), someday I may get creative and wire up my 120 so that the reverse switch locks out the o.d. and try 8 forward.

Anyhow, the 4th gear switch if present is on the gear box top cover, getting to it, ot even seeing if you have one will probably require taking out the tranny x member bolts and removing the shifter, and lowering the tranny as far as you can (loosening the engine mount bolts as well helps, just make sure you don't force the fan into the rad). Once you have done this you should be able to see if you have the switch.
If you don't you can scrounge around for a top that has the switch or you can get the measurements and remove your top and tap it for the switch, or you can wire something up so reverse switch disables the o.d., or you can use the 240 o.d. relay and put a switch on the clutch pedal so that pressing the clutch disengages the o.d. How many ways can you skin a cat?