The easiest and cheapest way i found to get a tachometer to work with a
4bt was to purchase one that can be used with a magnetic pick-up that counts the teeth on the flywheel.
Just drill & tap adapter plate directly over flywheel teeth and install pick-up.
Tach $16.00 New surplus. 3"dia. 0-5000rpm.
Mag p/u $28.00 Truck parts house.