It's Alive!
Well, everything is back together, gauges have been repaired and fuel system purged of any trapped air from the filter change or opening the injector lines during the glow plugs/comp-testing. Also, I checked the replacement tach sensor before I installed it, ~2.4k Ohm reading IIRC where the previous (bad) one was showing as an open circuit.
After clearing the arachnids that setup shop in the cab (4 in just a couple days, that time of year...) it fired right up and immediately saw that both the EGT gauge and the tach were working, high idle also appeared to work since I reconnected the sensor. Zipped to the gas station (only had a couple gallons of street-diesel to fill with after draining the tanks (which I have ~12 gallons of kool aid diesel to sort out) and didn't want to run it dry and be stranded after idling a couple minutes to get things up to temp.
Once on the highway, HUGE SUCCESS! 55mph is a bit shy of 2k rpm per the tach, but most importantly it was holding top gear without hunting between gears anymore; tach sensor definitely seems to have fixed that one so it'll stay an automatic unless I have reason to suspect the reliability of things. Boost gauge works, seemed to cruise 55-60 mph at 1-2 psi (hard to be certain since it isn't mounted (just laid on the dash at an awkward angle) and heavy throttle up an on-ramp showed ~7-8 psi. EGT gauge showed ~300 at idle and saw a high of 650 at WOT on the on-ramp with boost. The 088 ATS unit looks like it has the EGT sensor after the turbine on the downpipe side of the exhaust housing, so guessing the gauge is a bit optimistic/low, but I didn't see any numbers that scared me; freeway cruising was 450-600, depending on speed/grade/throttle.
Very happy with the purchase right now, time to continue maintenance and repairs to see what else needs attention.