I will second the Allison transmission suggestion (It is what came on my Workhorse)
I've heard a lot of things about transmissions... Had a few things to say about a couple of them myself (the kind of thigns that might well melt your monitor)
however ALLISON transmissions... Well, I don't have enough expierence to say anything personally...
But I've done a bunch of reading and researching and the if anyone has anything bad to say about them..... I've yet to see it.
Other companies.... Well let's just say I don't think much of selected Ford transmissions. (For example the Ford 4-speed RAD (Means Reverse Always Dies far as I can tell) a design flaw put a pivot in the wrong space so you only get about 10% engagement on the reverse gear causing it to wear out quickly
It is a manual however, not an automatic... I do like a big stick
Mine is a Ford E4OD (introduced first in 1989) hooked to a 460 cid-fi and has been trouble free. The overdrive is great. I would prefer a little higher stall speed, but that's the torque converter; not the tranny. The 5-speed would be an improvement to fill a (perceived) hole between 1st and 2nd gears.
I have a 2006 2500 HD GMC. I comes stock with 6 speed Allison. I haven't transfered my hitch to it yet, but putting it in tow mode and you can feel the tranny doing a lot of the work for you. Down hill or braking for the off ramp and you can feel the transmission downshifting to assist. The manual mode is something I look forward to. Put it in manual and you choose the highest gear that you want it to run in. What ever you choose is the highest gear it will go up to. That will be nice for long uphill grades.
I've heard nothing but great things about allisons and they do put them in A1 Abram tanks so they must know about heavy loads. The sales man said the Allison factory rep said the Park pin on the tranny is rated to hold back 17,000 lbs. Not that he recommended it, but he said you could park it on a hill and not set the brake with confidence.