probably the feature lamp bridge. it usually gets hottest by far in normal conditions.
These transformers found in later bally games run pretty high voltage to the feature lamps. A few games like kings of steel and black pyramid i have picked up had all the lamps blackened and mostly burned out when i got it. There was over 7v going going to the feature lamps. 1volt difference between the two games would result in a lot more heat at the bridge for the higher voltage one.
Stick a big hunk of finned aluminum to it for a heat sink. You can buy one from China for like a buck. Might have to drill a hole to mount, but it will help your bridge survive.