I suspect getting the correct ratios and spacing would be more useful than finding a specific trans. suitable for towing. Most any STD. trans can be towed safely, just always make sure it is in neutral, no kidding, it happens more than you think, make sure oil is full, heed advice above.

Ed

'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.