My understanding, perhaps wrong, was that 'BHP' was a legal measure, akin to taxable horsepower in early European automobiles, and has little real correspondence to lb/hr steam mass flow being generated by a particular boiler.
I don't think there is a practical solution - certainly not one I'd consider either adequate or 'safe' - other than actually testing the mass flow of steam through the valve when it is popped. That could be approximated from physical constants, but there is probably no substitute for empirical testing (of the kind prescribed by the National Board).
I think there is a practical issue in testing the 160lb valve directly on your boiler, as it would involve carefully balancing boiler pressure and injection while keeping the 160lb pop open and the 163lb pop closed for a time sufficient to get a measurement. I would not gag the higher-pressure valve to simplify measuring the 160lb mass flow!
It is simpler, of course, just to try the measurement with both safeties lifting, control the boiler pressure with the injector, and do any nit-picking mass flow corrections mathematically after 'dividing the result by two'. You can decide what kind of flowmeter you put in the injector line to measure the mass flow in, and how you keep the level 'determined' -- the gauge-glass reading may not represent actual level with the pops lifted, etc.
If it were mine, I'd do the test 'on the bench' or put the valve on a boiler with a known higher pressure rating with known adequate relief capacity through its own (separate) safeties, run at the appropriate lower pressure. Option 3 from the list Mr. Austin provided might represent the best of the protocols for a practical test in this situation. The National Board synopsis of state regulations is here:
http://www.nationalboard.org/PrintAllSynopsis.aspx?Jurisdiction=Select
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