On the 300B conversion, if you have B+ above 300, and ability to deliver 80 mA, you can suport it. You'll need the dedicated 5V filament TX of course. Possibly a bigger negative rail for a fixed bias circuit if B+ is on the low-ish side.

The 300B looks best operating into 3k5 per tube( IMO you see ). For me that means two 300's and a 7k PP OPTx, but we're talking your SE amp now...:)

It looks like you have an EH 6SN7 in there. I do most heartily recommend a NOS US made replacement. The black RCA is a hard one to beat( and not fanatically expensive ).
cheers,
Douglas