Whats wrong with the mb unit?? I installed an mb tach using mb sensor on my '84 6.2 diesel blazer. I had to replace the water pump one time & saw that the harmonic balancer had a small slit machined in it for a probe for checking rpm. the slit causes a disruption in magnetic field (very slight) same on mb's with sensor on front of motor. (I dont know about newer (85's+?). Anyways, while radiator, fan, etc were out, & new pump installed, I made & mounted a bracket on one of the w-p bolts to hold the mb sensor. Got power (always hot + switched) from glow plug setup next to which I mounted the mb tach amplifier (tach from inst panel, sensor, amp & mount bracket all from Ecology, about 30 bucks) Works like a charm.
Pretty much any motor with that slit on the balancer ( a LOT of motors have one, a LOT of chevy's do for instance, inc. gassers) will work with this method.
-c-