I would like to know what transmission and clutch y5.9L diesel in to my 1991 Chevrolet 3/4 ton 4wdou would recommend when installing a 1993 Cummins ?

We would recommend our New NV4500 its our part number MTCRY016N, this transmission has a 12 month warranty, your cost is $1895.00 outright purchase no core required plus freight.  This transmission will have custom tail to mate with your transfer case.  You will also need a 5.9 Cummins bell housing your cost $345.00.  Clutch/Flywheel kit our part number MWT1191SK Zoom Variable Torque your cost $575.00 plus freight.You can see the NV4500 units and parts we offer on our site.