Can't see why not.

Hard to say about the PT without knowing the plate voltages being used.  But a switch to a 5Y3 rectifier should lower voltages enough to use *at least* JJ 6V6's, which can handle higher voltages, and it may lower voltages enough for more traditional 6V6GT offerings. 

And the OT will need impedance changes with a switch to 6V6GT's.  It uses a multi-tap transformer, which makes this easy.  If you think you'll be switching between 6L6GC's and 6V6GT's, then you may want to connect the 4-ohm tap instead of the 16-ohm tap. 

The 4-ohm tap will be 4 ohms when using 6L6GC's, and it will be 8 ohms when using 6V6GT's.
The 8-ohm tap will be 8 ohms when using 6L6GC's, and it will be 16 ohms when using 6V6GT's.
The 16-ohm tap will be 16 ohms when using 6L6GC's, and it will be useless when using 6V6GT's.

Can you get it down to the stock 12-watt output of a 5E3, I dunno, I would need to know the voltages.