1986 GMC Astro rebuild...tach question

The Astros I have experience were older than yours. They had a 8V71 or 855. What I refer to could have been seen on construction, ag, stationary, or on road. The Astros had cable driven tachs. Some Detroits had tach drives off a gear cover just below and right of where your alternator is. Your truck probably has a pulley there that runs the alternator belts. Others had a tach drive off the blower. I was thanking it was on the left rear but that can be seen in your picture and there is not one.
Some vehicles run the tachometer off of the alternator like the Volvo FE7 and some New Holland tractors. I thank they operate off the terminal which reads about half of the alternator voltage output around 6.4 through 7.25. How they convert alternator voltage to accurate engine RPM I have no idea but maybe a supplier can set you up.
Some buses and RVs with these engines used an electric pickup connected to the mechanical tachometer drive.
If you do not have a drive I would be looking for a 3/4 inch fine thread hole in the flywheel housing where the ring gear is or make one. Install a tach sensor and a gauge with dip switches.

 

My tach ran off my accessory drive. That's what the black cable is

 

I always had good luck with Isspro tachometers. They have many different versions and sizes.
Electrical Tachometers - Page 1 - ISSPRO Inc.

If you want to use the sensor that is in the threaded hole and sensing the flywheel ring gear, you need MPU style tachometer.

If you want to use signal from alternator, make sure your alternator has a tachometer output (often called W-output). This will give out a pulse signal for the tachometer..

Number of pulses per revolution to be set with the dip-switches.

Example for MPU style 3 3/8 is R5535M
R5535M

Instructions for the different models:
ISSPRO.com - Quality Gauges, Tachometers, Speedometers and more!

Good luck