What plexi said (I skipped the bit about it being a vintage Super when I first read the OP)
I believe he is speaking of a clone for a friend. A transformer PT for the 5F4/6L6 has more voltage and will cause the plates on the 6v6 to run high, possibly too high. The OT could work well considering the wattage and 4 6v6. A 5k OT. Optimally, you would want 4k, but a replacement OT similar the 40 watt Super Reverb will work.
The 6v6 version sounds very similar until a good volume is reached. I have had both and now I have 2 5f4's with 6l6. One is stock, finished combo, tweed/Oxblood. The other is caged head and is modded some. Sounds great with 2/12. A 5f4 is the cleanest tweed amp, but I do not know what
your 6v6 version sounds like. It is very hard to distinguish power tube differences without a healthy volume.
If your friend likes the tone of your amp, but wants more volume you could simply use more efficient 10 inch speakers if yours has original speakers. For instance, 2 eminence Red Fang 10's will produce more volume with 2 6v6 than old Utah's or jensens. Much more, and that is why most desire more watts.
To answer you tho, I prefer 5881 Tung-sol USA, with a 5751 in my V1 and a smooth plate Telefunken 12Ax7 in the PI with fixed bias. A TMB tone stack with presence.