Buy an autometer diesel tach part #3788, clamp the pickup to the housing, follow the instructions and you are good to go. KISS, there is no need to re-invent the wheel.
I've had doors for my '37 GMC shipped via Greyhound Bus from CA to MA. I don't recall the cost but I didn't have heart failure. A bunch of plastic parts shouldn't be a big deal. I needed to pick up at the closest bus terminal, just a thought.
I have never had good luck with Dorman hoses, whether for a 6.5TD. 6.2NA, or a BBC. I ended up using AN fittings and braided stainless hose. Your results may differ.
You might be able to make an '81 or newer tray work. I looked at my '85 with dual batteries and the only difference I can see between it and my '74 are the stamped holes. Just a thought.