Sorry for taking so long to respond, i'm in the middle of moving, moved in (per se) but still living in the cottage until we can see the floor!lol
So, many ways to skin a cat, I would go to a schoolbus junk yard and get a tach from a same model bus if possible. Note the wiring on the back of the alternator and wire it in. Worst case trade him alternators if yours doesn't have the tach tab on it. Could also pull the whole dash panel and install the panel with the tach in it to make it look factory. My bus never had a tach so when the previous owner swapped engines he pulled the tach out of the 5ton truck he got the motor from and installed it in the bus as well. Regardless of how you do it, you should confirm with a hand held rpm counter. Any diesel garage should have one, it's a piece of silver tape stuck on the damper then you point what look like a timing light at it and it tells you the rpm. If it close, great, if not use fingernail Polish or liquid correction stuff to make the importand rpms.(1700 for lowest lugging point, 2000 for cruising and 2500 for the highway. Final one would be 3000 as a redline, yes it's below the factory redline but it's a safe cushion in case you get coasting down a hill and don't want to overspeed the engine....still on my BlackBerry and typing sucks! Lol.