A few related comments:
1. The screen current in the Ranger's 6CL6 is higher than you think it is. I don't know the exact value... but looking at the data sheet for a 6CL6, my guess is that it is closer to 5 mA, and possibly more. So, with (for example) a 300 volt value of the low B+, and to create a screen voltage supply that is reasonably "stiff" ... you would want the pot to be drawing around 10 mA from the 300 volt supply (with the screen of the 6CL6 drawing zero current). Thus you end up with 3 watts for the minimum dissipation rating of the potentiometer.
2. Using a few modern devices, you can substitute a low wattage pot. You need to add a transistor (and NPN bipolar transistor or an FET) that (to be safe with respect to some hypothetical transient conditions) can handle the 300 volt low B+ (actually, you should assume that it can be as high as 400 volts). Ref:
http://www.amwindow.org/tech/htm/drivepot.htm In the referenced article, and the associated schematic, there is a 6.8k ohm "existing screen resistor" shown. This resistor does not exist in the Ranger... and you might want to add it to reduce the power that the transistor has to dissipate.
Also, the label: "to screen of 5763" should be changed to "to screen of 6CL6" in the case of the original Ranger (a.k.a. Ranger 1).
Best regards
Stu