The diodes are a fail-safe in case the rectifier tube goes south. As has been said, 1N4007. A fellow at Ampage says abou the caps ...
The cap is not essential. It has been put there to help suppress the switching noise of the added semiconductor diodes. The switch noise will not get through the tube rectifier BUT it will couple back through the transformer to other windings which can cause the switching noise to appear on heater windings or worse still on a winding used for a bias supply.
The bias pot + resistor combinations are abundant, those two seem pretty popular. I would guess that you want your bias pot somewhere 'round the middle, with leeway both directions.
Filter caps can be all kinds of combinations ... certainly can't see much reason to have 2 X 20uF + 2 X 22uf ... all 20uF or all 22uF won't be much different
With the GZ34, you can go as high as 60uF and be in spec. You can go higher with solid-state rectification, but don't see the need unless you're a little shy on P.T. voltage. As has been said, more filtering = cleaner and tighter at the top.