Some good advice here so far.
The 200S was designed for KT88's/6550A's. Some of the earlier Sunn amps like the Sonic I-40 for example used lower voltages and different transformers and EL34's. With today's AC wall voltages at least in the US, the regular idle B+ voltages are well over the 525 can cap rating anyway. My 200S runs at 560v B+ on a 125v AC supply. There aren't any can caps made today that will allow you to have a voltage rating high enough for these Sunns. They will work for quite awhile on those higher voltages, but will fail sooner than if you had the right voltage rating. You
CAN change to series-connected discrete caps however, and get a higher voltage rating, and it will probably cost less too. A couple 350v caps in series will give you a 700v rating. The first two sections in the 200S for example should be series connected with a 700v rating, and the third section should be at least a 500v cap, or more series connected ones. The fourth filter section is ok with a 450v cap.
Using the 5U4GB isn't recommended because the rectifier filament tap may not be able to supply adequate current for that tube as it draws 3a and the GA34 draws 1.9a. The tap is probably only rated for 2A. Using the 5U4GB could cause the power transformer to overheat which will cause it to leak wax, internally short, and eventually die. You could use a Webervst Copper Cap solid state replacement rectifier, and that would lower the voltages like a 5U4GB, but doesn't use any filament current at all. They are $20 at his site, and you would want the WU4GB.
www.tedweber.com.
Using a good quality EL34 would be ok, but I would make sure to add some 1k 5w screen grid resistors. Most NOS EL34's would be best because they can take the stress of a UL circuit such as the Sunn. I would avoid Siemens NOS EL34's in this application however, as they can't take the stress. The new manufacture Winged C/SED EL34's are great, as are the JJ. As suggested by George, the resistors in the bias circuit may need to be changed to get an adequae voltage range adjustment to the bias circuit when using EL34's. You could probably look at the schematic for one of the Sunns that came with EL34's originally like the I-40 for examples of which resistor values to change to.
Using the Sunns as a guitar amp isn't something I would do personally without a lot of changes to the amp, but as a bass amp they are great as is, and can be made even better by using a solid state rectifier replacement like Weber's Copper Cap WZ34, and going with larger filter cap values in the first two stages in the amp.
Greg