Few random Q's about 318 and gauge wiring

15" Hg at 6900 MSL is pretty good.

Slack in the timing chain isn't the issue. The OEM cam sprocket has Nylon teeth which get brittle and come off with age. Result is bent valves usually.

you can do a cam swap with the engine in the car easy enough. Pull radiator and grille. Might have to jack up engine slightly.