Without knowing what the secondary impedance of the transformer is (there are ways, with hooking an ac supply to the tranny), it's safe with assume the double/half rule applies. You can double or half the speaker load without hurting anything. If your ot is looking for an 8 ohm load, you can run 2 speakers in parallel for a total of 4 ohms or in series for a total of 16 ohms without blowing anything up or shortening tube life.
I would run a couple of 8 ohm Webers for a load of 4 ohm, it seems to be the most typical configuration for an American tube amp of that vintage.
If you could, post the transformer codes from the output transformer and I'll see if I can dig up a manufacturer/model number.