Maybe a bit late on the reply on this:
No, a T5 would not carry the load. I'm thinking I should put a temperature gauge on the main box as when it gets hot the 3rd and 4th syncros bark if I shift to fast. This is since I put a Novak overhaul kit in it and switched to mineral oil rather than GL4. Never did that before but now it got a hot 261 in front of it.
As Jerry said, the main box is real truck transmission, an SM420 and the brownie is a progressive 3-speed, not a deep under, -- 24% under and 17% over plus direct. Says Spicer-Brown Lipe 6231A on the tag.
After 7 hours of running down the CA central valley at 63MPH, 2600RPM, the brownie creeps up to about 200 degrees. That's at 65deg ambient. It doesn't get that hot on a hard pull in under drive for whatever reason, probably because the output shaft is turning 2/3 as fast. Input shaft speed is still about 26-2800 RPM. With the previous engine, a similarly tuned 235, I would have to pull in 3rd and overdrive for short periods, maybe 5-6 minutes, over the Siskiyou Mtns. and over Tehapachi out of Bakersfield for longer periods. I worried about main box temperature but no gauge. With the 261 I can pull both in 4th under. If I put a gauge on the rear drive axle then I could worry about that too. I ran the 5.13 axle for decades at similar speeds, maybe 3 MPH slower, and it never complained. I used to get paid to do this sort of thing back when trucks had 2 sticks and 335 horsepower if you had seniority, 220 if you were a newbie.
I have used this rig to tow an 8,000 lb. travel trailer home from North Central Montana to North Central Washington. It wasn't fast but it kept ahead of the log trucks.
