Welcome!
Since this is an IDI, I would recommend to go to our sister site:
www.oilburners.net
The 6.9 and 7.3 IDI section is about the best forum for these trucks.
However, with the being the F-Superduty, the highest gear is going to be 4.88:1, but more than likely 5.13:1 or lower. The F-Superduty (450) is geared this way for GVW. That speed limit is about what you would expect for one of those trucks. They will run pinned to the governor all day and be just fine. But it's not going to be a speed deamon..... Your biggest limiting factor is the Dana 80.
Once you are on oilburners, search on user towcat and F-Superduty or F-450. He ran them for years, unfortunately he passed unexpectedly a couple weeks ago.