I've seen guys use the Federal Signal PA300 controllers for shows - pretty straightforward wiring through the existing radio harness if your Diplomat still has the police wiring intact.
Yeah I'd stick with 5/16" steel line for that application, you can get good pre-bent stuff from Classic Industries or just get straight sections from NAPA and bend it yourself.
Sounds like your blend door actuator might be stuck or the vacuum lines are shot - those old Mopars are notorious for vacuum leaks that mess with the HVAC controls.
Yeah I'm with you on the confusion - seems like there's conflicting info on whether the J-body changed dimensions in '80 or if it stayed the same as F/M bodies.
Been running a Sniper 2300 on my 318 for about three years now and it's been solid, but like others said you're gonna be deep into it with fuel tank mods and new lines - might be worth fixing whatever's wrong with your carb setup first.
I'd go with the solid aluminum spacers from Firm Feel for the front too - once you feel how much better the rear is without the isolators, you'll want that same solid feel up front.