Use wing-nuts on the bellhousing studs, so you can easily remove the transmission every time you shatter it. If the gauge cluster is anything like my '88, you'll still need the speed sensor in the trans to operate the speedo, odometer, cruise, and anti-lock brakes.
You'll have to dig into the schematic to get the tach to run from the Tach terminal of an old-style HEI.
At some point GM switched from old-style gauges to stepper-motor gauges. Far as I know, all the stepper motor gauges are driven by the computer. Disconnect the computer, gauge cluster is dead.
I wouldn't trust the rear axle, either; although the 8.5 would probably be fine if you don't have a Gov-Bomb and you use narrow, crappy tires. I really despise the leading-trailing shoe rear brakes that come on the 1500s. When it was me, I found a light-duty 2500 that had a 9.5 semi-float and decent rear brakes. No idea how that's gonna work with five-lug front wheels.
How are you going to regulate fuel pressure?
What are you using for an exhaust? I assume you're scrapping the catalyst while you're vandalizing all the rest of the emissions system. I thought NY was tough on emissions. I guess I'm wrong, 'cause there's no way you'd pass an inspection.
Bigger radiator Probably do not need the engine oil cooler; but it wouldn't hurt.