I'd have to differ with you on the ZF transmissions vs the NV. The NV4500 is basically the old NP435 with an OD unit added to the rear. The gear ratio spread in 3rd and 4th are not ideal. Works OK in the 6bt but that engine has a lot more torque than most 4bt's. As far as smooth shifting, I've driven a ZF S5-42 320,000 miles with zero issues. Not sure where you got your prices on adapters but $3K is way too high. A lot depends on which ZF you plan to use. The SB Ford Cummins adapter/flywheel will work with the SB ZF. To use a diesel model like came on the 7.3 which has the best gear ratios will require an aftermarket kit which is in the $1500-1700 range.