I have to ask....has this Ampeg been recapped. IF not, you are barking up an empty tree, ime. IF it has already been been recapped, there are other ways that are much simpler to get more gain out of it....biasing, preamp gain changes... IF you like the basic tone of the amp, I would not suggest changing to a different type of tube. You will change the gain, but you will also drastically change the tone.
to answer your question, though, I don't know of anyone who builds octal to noval converters for 6sl7 to 12A_7.
I just took a look at the schematic. One gain stage prior to the PI....I simply would get it healthy and turn it up to 10. That is when these types of amps go to work, ime.