According to the power supply schematic from IPDB - http://www.ipdb.org/files/762/Bally_1981_Eight_Ball_Deluxe_Manual.pdf

If the machine is set to 120 V, the transformer jumpers are set to ;

Pin 2-8
Pin 4-6
Pin 7-11

(There will be two jumpers 5-6 & 6-7 already there)

For 240 V you will need to change the transformer jumpers to the following ;

Remove Pin 2-8
Remove Pin 4-6
Add Pin 4-8
Leave Pin 7-11 (if already set to 120 V)

(There will be two jumpers 5-6 & 6-7 already there)

Please be sure you know how to read the transformer pins. They should be already clearly marked. The schematic I'm reading from is on page 50 of the link I've provided. Double check your work.

Check the value of the Varistor (on the filter located in the cabinet) - it should be rated to 250 VAC. You will have to change it IF it is rated to 120 V.