FWIW, I have the original Y type and before my allignment, my Castor was 4.2, which I think most trucks are set to at the factory.
They set mine to 5.2 (the high end of the spec) and while the truck is VERY stable at speed and has excellent return-to-center, it also want's to turn away from the crown in the road a lot. I need to put some towing miles on it to know for sure but at this point I'd rather he had just set it back to the orig. setting. It's not a huge deal but I think I preferred it the other way.
I don't know if castor effects the T type steering as much (I would think so).