old F250 Suggestions

[ElkinsPark]

Just bought a 1991 F250 7.3 diesel, naturally aspirated. Never drove a diesel before but have had a few in the family. It seems like a decent old truck overall, just old.

I began pulling a mobile coffee shop plus it's equipment, about 6500lbs, across the country with it, from WA to PA, but the transmission overheated and I'm "stuck" in Missoula. I like to think of it as an unplanned break... The shop here is putting in a rebuilt tranny but now I'm a little bit afraid of stressing this old truck out again. The engine sounds strong but I haven't had any legit diagnostics done on it. It allegedly only has about 118K miles on it.

Any suggestions for driving an old truck like this across the country, pulling this kind of weight??? I'm heading to Philadelphia with it, and now thinking of just driving really slowly for most of the way (50-60mph) for maybe 8-12 hour days. I only have one more big pass to get over, so I hear. I guess I'm just not sure about how fast and how many hours per day I can drive a truck like this, for how many consecutive days. Any advice is appreciated.