High torque manual gearboxes

What's the highest torque anyone has running through a manual gearbox? I've got an American truck engine sitting in my shed which is just begging me to swap it into a Landy and make a stupid weekend toy, but the problem is it makes about 300lb-ft at 1200rpm and heads up to 425lb-ft at 3000rpm.

I know there's a huge number of autos that could take that no problem, but I'd prefer to have a manual for the fun of it. So far my searches for gearboxes that could take the torque seems to be limited to big American pickup gearboxes (which are hard to find here) or luxury diesel cars like BMWS (but the ratios aren't great for low range).

Anyone got any suggestions?

A lot of it will come down to how you drive tbh. The old Series LT-76 has a very low torque rating, yet loads of people have used Rover V8's and Tdi's with them.

The R380 is rated at 380nm, about 260ft lb. Land Rover didn't fit these to the 4.6 RV8 as the 4.6 was over the torque rating. But lots of people have raced with these boxes over the years. There is also a company in Oz that do an R380 to Chevy LS1 adapter plate. So there must be some people using this setup. There are also M57 to R380 conversions out there too. If you abuse it, it'll probably fail or at least sooner. But I suspect it might be fine otherwise.

The LT-85 is technically a stronger box, but the gates are miles apart. It is a very clunky and slow shifting box. I don't think it would be the way to go.

The MT-82 from the Puma is a Getrag/Ford box, also used in the Mustang GT, so should be capable of higher torque outputs than the Rover/Santana boxes.