KB, all those SUnns of that era are very similar. The amps with EL34s were nominally 40 watt amps while the more or less same model with a pair of KT88s was a 60 watt. You can tell which it originally used by the transformers. The output tranny was 3-40T, 3-60T, 3-120T, the number corresponding to the output power. so if it has an OT that is 3-40T, then it started life with EL34. If it has 3-60, then KT88s.
Same with power tranny. 3-4PT, 3-6PT, and 3-12PT. The 4,6,12 were paired with the 40,60,120 respectively. So if the amp has a 3-40T OT then it ought to have a 3-4PT PT.
The 6AN8 and 7199 were similar tubes, though the pinout differed. The 7199 was paired with the EL34, and ther 6AN8 was paired with the KT88s. SO if you have a 7199 circuit, but KT88s, I would bet someone stuck the KT88s in later in life.
The tranny numbers should tell the tale 7199 and 3-40 means EL34s.
If you intend to replace both trannies, I don't see why you couldn't make it appropriate for 6V6.
Since you are replacing the PT, The B+ is going to be whatever you make it. You can always alter the dropping resistors for the following B+ nodes to make them whatever you want. SO I see no reason the PI stages can't be at the original working voltages with little effort.