The switch on the NP435 was a screw in affair in the top of the trans near the reverse shift rail. Clean off the top of the trans, you may find the spot for the switch just has a plug in it. If it's not drilled out it shouldn't take too much to swap the trans top with one that has a switch.
Backup lights were a separate light on all bed styles to 71 and the Utilines past that. The originals are real hard to find in good shape, but the $5.99 generic backup lights from Checker or O'Reilly look and mount real similar.
While your doing all the wiring, use a single pole double throw switch on the dash in addition to the switch on the trans, you can wire it up so center is off, one way is automatic so they work in reverse, and the other is on so you can use them for work lights. To wire the switch, hook the center pole to the lights, one of the outer poles to the backup light switch and the other to a fused power source.