The TKO 600 has a provision for a neutral safety switch but it is a low current circuit that signals the computer (in later model cars it was designed for) that the transmission is or is not in neutral. I'd like to use the switch to send a 12v signal to activate a relay that will be wired in the starter circuit using the purple wires from the original NSS. Will a 30A relay be enough or do I need to step up to a 40A or even a 70A?
A 30 amp switch should be fine to activate the starter solenoid. I hooked a remote start system up to my Skylark for a while (back when it was carb'd) and ran the remote start through the TKO's neutral safety switch and did just like you are proposing, using a relay to power the solenoid with the signal coming through the TKO's neutral safety switch.
The switch in the transmission has a low current capacity. If I tie in directly to the starter circuit will go up in a puff of smoke.