That's a nice tractor. You just made the cutoff for the new and improved transmission at SN 94N5192, and I've heard that's a good thing. I'm pretty sure yours is a 1972 model but I don't have the book.
That's a 1973 model and the transmission change occurred at s/n 94N5660. Prior to that it used a D6C transmission that didn't hold up well.
Yes, the 3306 was introduced at 94N5192 and the transmission upgrade was at 94N5660 from the D7E's transmission to the 2P transmission that they adopted from the D6C which was better suited to the higher rpms of the 3306. The earlier transmission was held over from the previous model which was a lot slower revving engine (D339) and couldn't take the higher rpms with heavy work over thousands of hours. I doubt you'll be putting thousands of hours on yours every year so I don't think it's a big issue.
The main advantage to the transmission upgrade was the increased number of clutch plates. The tractor itself has a stronger undercarriage and frame plus a 7% added cooling capacity.