Car shifted into gear when in park...

I recently got the 440 and 727 installed in my Barracuda. It's not running yet, but as I was installing the driveshaft I discovered that it wouldn't go all the way into park. Fought it for a while and finally looked at all the parts. Trans is a '67, but due to the shifter I'm running (late 70s slap stick) I needed the three prong NSS/backup light switch. My '71 valve body was a crusty mess so I used the '67 body. Turns out that the late rooster comb doesn't work without some grinding away on the early valve body. This is the first time I've tried to use this combination of parts and I had read that you just swap the rooster comb to use the three prong switch. Now it stays in park.