Yes, to answer the question "i'm wondering if it could be hooked up to a regular tach gauge from a 300D" - just do like I did.
It is not a lot of trouble actually, the new tach/clock will replace the existing clock now in the right side of your cluster.
Finding a working one is the hardest part. I bought one at a junkyard that was no good. Another had a good tach but the clock was dead. There is a fix for the clock, its simply replacing the small electrolytic cap located on the circuit board, they dry up and short.
Dieseldiehard