Thanks for the schematic, it helps somewhat. But that scan is hard to read. Can't read the parts values, connections, etc. Can you get a better scan, or re-draw, and post it? It looks like there are a couple of low pass filter capacitors in the input plate and driver plate circuits. Why, is this a 300B Guitar tube amp? Or is it a Hi Fi amp? Or are those capacitor wires just crossing, not connected (can not see enough detail). Since you will now be converted to self bias, you can change the 47k 300B grid resistor to 100k, and ground the other end of that resistor. That will give a better bass frequency response, and a little more gain and lower distortion from the driver stage.
I can not see the details of the pre self bias, or the post self bias circuit. I suggest DC on the 300B filaments. It will eliminate the 2X line frequency intermodulation that occurs on each and every musical tone. With AC filaments, Hum can be adjusted to be low, but the 2X line frequency intermodulation can not be nulled out. I can not see enough of the circuit details to make any other verification of the circuit, or other suggestions.