Most of us Singlepower owners have been using adapters for a couple years now .... they work great. But, I would check with the builder to make sure.
Currently some of us have been experimenting with a 6SN7GT to 6F8G adapter. This tube is essentially a 6SN7GT with a plate cap.
Here is a pic of the tube in a Supra ....
Where I am going with this is while I was researching the 6F8G I discovered the 6C8G. The 6C8G is essentially a 6SL7GT with a plate cap. However, this tube while almost identical in most electrical parameters to the 6SL7GT has a noticeably lower gain/ mu of 36 .... compared to the 6SL7GTs documented gain/ mu of 70. So, you might want to ask the builder about this one too.
The 6F8G sounds as good or better than any on my 6/12SN7GTs and I suspect the 6C8G will give the better 6SL7GTs heavy competition too. As a plus, the 6C8Gs I found available were the better nos Ken rad/ RCA/ TS and NU types .... and these tubes cost a whopping $3-4 each. If the lower gain is sufficient this tube is really good. I tried them in my Supra-Extreme and even though the electrical environment was less than optimal the tube still sounded very good. In an amp made specifically to use the 6SL7GT these should be about as good as it gets.
I bought my adapters on ebay ....Adapters 6F8G TO 6SN7 6384 TO 6L6 tubes SUB - eBay (item 250332454155 end time Dec-25-08 18:45:54 PST)
Another option is to use an octal to loctal Singlepower 7N7/ 7AF7 (= 6SN7GT ) adapter with a tube called the 7F7. The 7F7 is a 6SL7GT with a loctal base.The 7F7 will work for sure as this tube is electrically identical to the 6SL7GT. 7F7s run about $6-8 per tube.
You can see the 7F7 data sheet here ....NJ7P Tube Database Search