WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
It is not that critical. For 6L6 (which 807 also is) the recommended load resistance varies from 2k5 to 6k.
The plate voltage with 6k5 should be some 450 V to get optimum results.
Then the screen voltage should be limited to 200...250 V.
If only some 300 V is available as +Ub, then the load resistance should not be more than 4k5.
I have used Edcor's GXSE 15 W series transformers with 15 W 811A SE with excellent results,
but obviously CXSE 25 should be even better, but it is essentially more expensive.
The reason for this is not the amount of iron, but the amount of copper and other things.