The 375 lb figure for the relief valve is not the pressure of the propane. The operating pressure of the valve is based upon the design MAWP (Max. allowable working pressure) of the tank, derated by an adequate safety factor. I haven't taken the trouble to look up CGA specs for portable propane tanks to determine the derating factor.
As said previously, you should extend the line after the regulator. It's a lot more forgiving of construction mistakes.
Safety relief valves are generally rated at 1/2 the burst pressure of the vessel and/or 2x teh working pressure under typical conditions. If you must extend the HP side, plan for 375 working PSI and you should be fine. Plus, the SRV will back you up in the event of an overpressure. Can you plan for 200#? Sure, but if that thing sits in the sun and hits 110°F you may have a failure.