The 6V6 and 6F6 is pretty much interchangeable, so the standard output
transformer should work. 500 ohms is not a common output today, although
the pp 6V6 to line output may get you in the ball park. It should use a
standard interstage transformer.

A push-pull old Hallicrafters transformer would work, since they had a 500
ohm output, too. SX-28, SX-62 and SX-42 all would have the right
transformer if you can find one being parted out.

73, Colin K7FM