Won't crank in Park but will in nuetral

Purchased a 72 Swinger was a 6, put in a 383/727, put all the wiring back in the engine bay. checked the NSS in the trans. I get a reading in Park & Neutral when I try to crank in park the seat belt light dims but won't crank, if I go to Neutral the seat belt light goes out and it cranks. Shifting the lever from Neutral to Drive with the key on, a relay under the glove compartment will click, what does this relay control? There's no butchered wires in this car. Thanks if anyone can help