The zf transmission which will work for your application is the SF-42 model which was produced for Ford trucks from 87-95. This transmission was only rated at 420ft-lbs of torque(thus the "42"). It might be ok if you're not drag racing, doing heavy rock-crawling, or towing a huge load. They are pretty tough trannys. The SF-47 model was produced starting in '96 for the modular engine and will NOT work with a 460 without custom adapters(has an integral bellhousing). This was rated at 470ft-lbs of torque. The newer ZF 6-speed is rated for even more torque, but you still have the modular bellhousing problem. Perhaps a solution is to go with a NV4500 or even better a NV5600 6-speed with custom adapters. Both of these trannys are rated for more torque than the the ZF SF-42. Another option is going with a Richmond OD 6 speed tranny if it's a 2WD(or 4WD if you want to try to rig a remote transfer case setup).

This is a copy of my earlier post:
I am doing a very similar swap. I have a '71 429 with a 1990 ZF 5-speed. Ford changed from a internal balance to an external balance big block midway through the '77 model year. This means that the later model flywheel is NOT "neutral balance" like the earlier models. Somewhere along the line, Ford also changed from a 3 finger clutch set-up(6 pressure plate to flywheel bolts) to a 4 finger set-up(8 bolts). So what you need is a neutral balance flywheel with the later model 4 finger clutch/pressure plate bolt pattern. You have a few options: you can use the early model flywheel(make sure it has the same O.D.) and have it drilled/balanced to accept the newer clutch/pressure plate assembly, or you can do what I did and buy a later model flywheel, have the weight eccentric machined off and put a neutral balance on it. Other options include balancing the flywheel with the crankshaft(or even better the entire rotating/reciprocating assembly) or looking into compatability of the older 3 finger clutches with the zf input shaft.

Hope this helps
mckst