Or The Output Jack

Or you could eliminate one of the speaker jacks and put the pot there on the back of the chassis. I have an old Marshall head that's been modified that way and it looks fine.

You might want to experiment with the master volume implementation just to see how much it tends to "gut" the sound. Master volume controls can tend to act like a low cut filter with a turn over point that gets higher and higher in frequency as you bring down the master volume. This can be good or bad for the tone so it pays to experiment. If you're saturating the preamp section and pulling it down at the master having a low end roll off can be a good thing.