Andy,
My first choice goes for a chamber cast!. But regarding the use of headspace gauge, for such "obscure" calibers and "unknown" or "in the dark" chamber condition, I do really prefer, and it's my own personal choice, to use at least, a shoulder locating gauge together with the rim headspace gauge. On pre-1920 (about that, it's not sutck in concrete and also depends on who was the manufacturer) the chambers are often longer than what they should, and base diameter is, quite often bigger than what it should be. This comes from my personal experience of older firearms, and you'll never see me shooting such a firearm before a chamber cast....
Regarding the pressure, since it's very hard to get the truth regarding pressure, we can only speculate about that. Pressure is not a simple thing in a gun; it depends on bore diameter, throat lenght, chamber measurement etc.... If let's say Norma says their loads are at XXXX bar (or PSI) they already added (generally 15%) extra to their data. So, you should never base your load on the velocity they announce; 1500 fps in their test rifle for XXXX pressure might be the same as 1275 fps in yours... for the same pressure. Also, and it's a personal choice, that's why I genrally keep my reloads, for older guns at no more than 85% of the maximum announced load, even if no pressure signs are presents.