For a pentode, the 6V6-ish 1619 is a good valve. I linked its sheet below. It has a few very useful properties for this project. One is its filament. It is a 2.5V/2A directly heated cathode. Very similar to the 2A3. Matter of fact, change the sockets from the 1619's Octal to 2A3 UX4 and you're off to the races.

Next, it can easily be supported by the Guinevere power iron on a mono-block basis. Since we're likely using a custom OPTx, it will be fairly easy to arrange a means to take full advantage of both types of final.

New production 2A3 is fairly easy to come by, and NOS 1619 is even less expensive. Both will yeild similar output power, at between 7 and 10 watts.

Any design specifics we need to include? It seems that this ought to be as flexible a project as is possible so that changes can be made as testing and tweaking require.
regards,
Douglas