If it were me, I think I would use two resistors (of half the needed wattage), and put them in the plate lines of the 6x5. Those were/are a problem tube.
I don't have the sams, but usually the 6x5 filament is run from the same 6.3 volt winding as all the small tubes in the set. If this is the case, usually one side of that winding is grounded, and therein lies the problem.
In a typical 6x5 setup, there is full b+ on the cathode, and the filament winding is grounded. A typical failure is a heater-to-cathode short that burns out the power transformer. It wouldn't surprise me if this is what happened to the original transformer.
The main thing is that white goo on the filament has to hold back b+. A 6x5 apparently wouldn't even hold to it's rating, let alone more. IMHO putting more voltage here is a really bad idea.
If the 6x5 has its own 6.3 volt winding that floats above ground, you can disregard all this
John