For the temp/pressure gauges you can use the stock senders from the gasser motor and get fitting to screw them in where needed.

Tachometer is an issue I can't touch too much, but some diesels have a tach output on the alternator that will work with traditional tachs. I'm using an autometer tach with alternator strap on thingy (diesel specific).

Speedometer - a VSS is normally on the output of the t case and if you don't swap axle gearing you should be good to go as long as the new tcase has an electric output. If not, conversion boxes are $30.

If you get a 90s that had a 6.2/6.5 diesel in it you could swap that alternator (with some bracket work) onto the cummins and have the factory wiring hook up normally keeping the factory tach as well. Then the sending units for pressures / temps swap in and hook up factory wiring to them as well. Starter and fuel shutoff would be your only worry but modifying the factory loom could get you hooked up quickly.

That sound easy I'm just not to familiar with the chevy wiring because I work for chrysler. Thanks for the info