I just replaced a factory posi for an Eaton in a 67 belvedere. The guy races on Wed night and he tore the spider and side gears out of it.
Let me know when you are going to the track. I will bring my Gopro camera to mount in your car. Also if you can weld an 02 sensor bung in your exhaust we can hook up my wideband 02 sensor meter and dial in your carb. Steve