I have a MB Electronics upgrade tach amplifier in my 300TD, works great. It looks like they have a solution for engine swaps here:
OM617A Tach Amp for Engine Swap Applications
Other people have also used Tiny Tach. When I do my swap I would like to make the stock tach work.... I bought a 1986 Jeep Comanche last march that is awaiting a 617!
Design Technology, Inc., Home of the Tiny-Tach - Diesel Tiny-Tach Tachometer