Yeah you can do it no problem. The two power amps run independantly, so all you need to do is adjust your bias at the bottom of the amp so you get about -42v on pin 5 of the two 6l6's. After that it would be a good idea to run your 8ohm spkr out to a 16ohm spkr cab, otherwise you will have mismatched output impedance(EL34s want to see 2.2kohm to 3.5kohm primary output imp. max, where 6l6s want between 4.5k and 6k, depending on your plate voltage).
Sorry to Hijack this thread. JP3 for your wise opinion.
How about the early Marshall 9100 older brother and it's an equivalent to the 50/50 EL34.
Originally it uses 6L6s, but it can handle the KT66 tube as well. But I wonder if it can handle EL34?