I would highly suggest RFT ECC83s for the preamp if you can find them. They cost a bit but preamp tubes last a very long time and you will have them for a while. If they are more than $40usd a pop, then you may want to consider buying new preamp tubes instead of NOS (new old stock).
If you are forced to go with a new tube, I imagine you would like the Electro Harmonix 12AX7s for the tone you are after - they have the best balance between gain and full sound than the competition. They have more gain than JJs, but not quite as much gain as certain Sovteks or Chinese tubes, and probably fuller sounding than anything else out there and they have always sounded great in Marshalls.
For the power section, you could go with either 5881 or EL34s.
If you want to stick with EL34s, I would probably go with the new Mullard EL34s without knowing specifically what metal tone you are after. If you want a more typical Marshall metal tone, then go with SED "Winged C" EL34s (NOT SVETLANA, they are not the same). The main difference is the SED EL34s have more mids.
If you go with 5881/6L6s, the best 6L6 on the market currently is the one 6L6 that Groove Tubes makes (and not rebrands). If you cannot find those (or they are ridiculously expensive which they can be) then go with the SED "Winged C" 6L6s. The tonal differences are not too big but the Groove Tubes made tubes will last longer.
Something that is very important though - be sure to have the amp properly biased by a tech. JCM 900s came biased way too cold from the factory and the sound lacks, but when properly biased they can sound great!
The other thing that you should have a tech do when you have the amp in is replace the heater with a heater with feet. After extended periods of use, the stock heater which rests against the board will generate a lot of heat and eventually burn right through the board and short.
