If the t-stat craps out with the sender located on the housing, then the engine can overheat. I have never seen a set of headers that block the sender from being located on the head.
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.