Yes two 6L6G or GA would be in the ballpark. The only concern for the OT is if it could handle the added current but from what you have described of the original amp, the OT was probably handling a high-ish current anyway. That is correct. It seems the 5Z4 from different sources say similar but different max ratings. I think the 5Z4 can take it but... I bet you will install rectifier backup diodes (recommended for most of our amps). The backup diodes would allow much higher current without issue. 40uF is the max suggested cap for the 5Z4. No problem using less. My opinion: the tube datasheet is usually very conservative and the tube can take it. As mentioned just use the backup diodes. Win win. I have found this tube reference handy. Quick and easy compared to individual tube datasheets. For the 5Z4 it is right with the purple line 125mA. https://frank.pocnet.net/other/docs/MK-RT.pdf The PT you choose may need to have a higher current rating to handle the two 6L6 and the ebay find will probably get you there.