Yeah, the pressure relief valve is there to keep things from exploding not to keep the system below a working pressure. the pressure switch should keep it below the working pressure.
Pretty sure that a tank rated at 140PSI is gonna be OK to 185.
SAE and AN flares are not the same as you've discovered. AN fittings were developed for the aero space industry specifically to have a low tightening torque for a good seal, and to be simple. Not sure why they departed from the SAE specs for the flare, but the aftermarket picked it all up since it is a very easy to work with system that works well with automotive fluids and supports customization. I bet a wikipedia search would answer the question better.
I'd bet a whole lot of money that anything stock is not going to have a flare that is compatible with a AN flare.
Anyways, they make nice 3/8"NPT to AN adapters that you could use if you wanted to. I used 3/8" NPT fittings and a leader hose from Kilby I think that had 3/8" pipe threads on it.
I bet you could find adapters from and AC place to take the SAE fitting to anything you want probably.