You'd have to determine if the power tranny could hancle the heaters, they draw about 50% more current than 6L6. Plug them in and leave the standby OFF so there is no B+ on the tubes. Now measure the 6.3VAC heater voltage. If it is still up to 6.3 or so, then the transgormer is handling it OK. If it is low, then it is not.
You'd have to make sure pin 1 of each EL34 socket was wired to pin 8. And not wired to anyting else.
You'd have to modify the bias supply to adjust to voltages the EL34 needs if it doesn't already have the range.
You'd want to make the screen resistors 1k 5watt types.