Without going to a medium/heavy duty application transmission which are huge you are likely limited to the ZF S6 they come in 2 flavors the 650 and the 750,
neither of these is rated to 1,000 ft/lbs but should do the job as long as you do not get heavy footed and drive them in a smooth fashion.
A B series Cummins will eat most any drivetrain component if you do not use a bit of wisdom with the application of power and limit shock loads.
I run a ZF S5-42 behind my 4BT with compounds that is in the neighborhood of 650 ft/lbs and it is serving me well but I just let the Cummins do what it does so well and am pretty easy with the clutch/throttle interaction, I am even running a HD D-44 front and D-60 rear with good results in a rig that is over 8,000# unloaded.
With the abundant early and smooth torque there is no need to get over zealous with the skinny pedal just be smooth and let the engine do what it does so well...$.02

ZF S6-650, S6-750 Transmission Information

ZF S6-650 and S6-750 6 speed manual transmission specs, gear ratios, and technical information. The heavy duty ZF 6 speed transmissions are found in the 1999 to 2010 Ford Super Duty Chevy/GMC trucks from 2001 to 2006.

www.dieselhub.com

 

The Eaton FSO8406A is a 6 speed manual that is rated for 860 lb ft. That one has a .78 OD. It's 25.6" long and would need an SAE2 bellhousing on the engine to get a clutch heavy enough for that torque. The FSBO9406A is rated for 950 lb ft. It has a .79 OD. I believe it's the same size, just stronger. That's the strongest transmission unless you want to get into something that came in a semi truck. Now, what one of those Eatons would cost I have no idea. Where you are they may be as rare as hens teeth. My guess would be in the $4000-5000 range and that may not be enough. You'd need an SAE2 bellhousing, flywheel, starter, clutch, transmission, and transmission bellhousing. You'd also need some serious size driveshaft parts and a super strong rear end. One rear that would come to mind would be a Dana S135 that came in Ford F550 trucks. That is one serious rear end. Ford down graded it to a Dana 110 in later years.

 

Perhaps, you can enlighten us as to what engine, vehicle and use, that requires 1000 ft lb of torque.

 

I suspect it's just a show thing. Few people drive around every day with 1000 lb ft of torque unless they are using a semi truck. Guys who run these engine with huge power capability seldom use a manual transmission. Usually, the big semi trucks can have more than that power but have monster transmissions. One factor most don't consider is that rebuilds will come more often on our smaller engines running huge power outputs. Not nearly as hard on a huge engine in a semi vs a smaller engine is a pickup truck.

 

Well considering that ram no longer offers a man trans anymore, the last offering in 2018 was a g56 behind the 6.7 that had 370hp and 850ftlbs tq in the standard output and 385hp/930tq in the ho units, I believe the 25 rams are up to 430hp/1075tq, this would be outside the g56 recommended inputs, so this could be a reason why ram stopped offering man trans, though ram claims lack of sales, not believing that one personally... the g56 would be my choice for ease of swapping, they are big units however and spendy... wouldn't mind having one in my future w250 build, just don't see many used units available, I'll probably just use the 47rh sitting in my shop...

 

I suspect the medium duty transmissions might hold the power but you have a clutch to consider. You'd need SAE2 as minimum or maybe SAE1. It could be done but not very cost effective. Of course, a high grade automatic is not cheap either. Companies charge in the $5000 plus range for something to stand that power.