In dash cluster aftermarket tach replacement-69 Cuda rallye cluster

Anybody done this? I see all kind of posts about replacing the ammeter with a voltmeter in the dash cluster, but none about changing the tach in the middle hole. Mine has a vac gauge now, but I know a tach was an option that year. I'm not going drag racing, so I'm not going to keep the 3 inch tach that was clamped on the steering colomn, lol. Still would like a tach for troubleshooting and tuning. And no, I don't want to spend $200 bucks on a gauge.