We spoke via email earlier. Mine measures out at 4 ohms, which would make it a 6 ohm speaker. From our discussion, I would go with an 8 ohm speaker, but finding one that would have a speaker basket shallow enough to clear the chassis is the bigger issue. After some thought, you really have three choices.
Find a speaker the same size as the RCA (difficult), have a new baffle cut and move the speaker as low as you can (easy, but no guarantee that you'll have enought clearance after all that) or build a tone ring adapter and install a 10" speaker(easiest).
Unfortunately, the RCA 12' speaker in there is more of a home hifi speaker than a guitar amp speaker and since yours has been converted from the stock 6-8 watt circuit 50L6 amp to a 15-20 watt Princeton/66J amp, reconing the speaker just means that you are going to blow it again in short order.
Then again, if the output transformer was changed at the same time that the power transformer was installed, then your amp will probably be looking for an 8 ohm speaker anyway. I would make a tone ring and audition 10" speakers.
My humble opinion.
