With triodes, single-ended, use Rl= 2*Rp to 5*Rp. Push-pull, go to the lower end, which actually works out near the sum of the plate resistances. Triodes are not too fussy.
Pentodes have a knee. For maximum power output, start at your proposed B+ and draw a line as far up into the knee as possible:
"Optimum" for 6BQ5 seems to be about 1K8 per plate, or 7K2 total primary. However the two other curves are nearly the same power, and give 5K to 10K primary. Since your speaker really varies all over the place "8Ω" being 6Ω to 20+Ω, no great precision is justified. 6K, 8K, and 10K are all standard parts. Each will sound a little different in a specific speaker, but different again in another speaker.
Oh, that chart is computed for 250V B+. If you change both Plate and Screen voltages the same amount, the relative V/I points all shift the same way, so 5K-10K is still the "optimum" for 6BQ5 with higher voltages on both plate and screen.
Note that 350V is over the book rating on 6BQ5: it won't burst, but may die young. This is often done in guitar amps: over-working the tubes gives more power while staying portable, and unlike home radios a good guitarist gets paid enough that tube replacement costs are not a big problem. Do you want a 250 pound Ampex amp that will run 30 years on the same tubes, or a 40-pound Fender that needs $10 of tubes a year? And I think most new-production good-brand 6BQ5 will really eat 350V just fine, since the main market is in over-volted guitar amps.
