Hi Henry ,
Were this mine to try , here would be my starting point ;
I would disconnect C7 from where it connects to C12 , R9 , and the #57 tube's plate , and then would connect a 1 Megohm audio taper pot with what your calling the "low terminal" to the place where you disconnected C7 from . C7 itself would go to the "sweep" terminal and the "high" terminal would go to ground . And that would satisfy the pot's three required connections . I believe before I started I'd take a measurement of the #57 tube's plate voltage , make note of it , and then check it again with the 1 Megohm volume control pot connected , it'll likely be lower due to the additional 1 Megohm load on the circuit so to preserve the correct plate voltage the resistance of R9 will need to be decreased somewhat , it's a 250 K ohm now , maybe in the neighborhood of 220 K or so for a starting point should get the plate voltage back up where it belongs .
And remember , "breadboard" this first , make no final changes till you see that the circuit actually tolerates the changes (no weird effects to the operation of the #57 tube with the additional 1 meg plate loading) and if it performs to your satisfaction in bench testing then go ahead and actually mount the new volume control .