44 Bulbs get very hot. You can see debades of warped plastics because of this. 47s will reduce the heat a bit and draw less power.

As far as the voltage is concerned there is nothing between the transformer and the lamps but fuses and sockets. Test resistance on the fuse in the holder from one side to the other on the solder tabs. This should be close to 0 but can get higher if the fuse and the clip are tarnished. A hint that this could be an issue would be warm or hot fuse clips.

Then pull the fuses and measure across the fuse clips yellow to yellow. These fuse clips are connected to 17 and 19 on the transformer. This is probably the bottom fuse clip to the one next to it. Make sure DMM is on AC and you should expect 6.3VAC. Might be a little higher without any load. If it's too high it could be the transformer. Check to see if it looks melted or toasty at all. The test from 17 to 19 on the transformer. If you get the correct reading there then you know the problem exists between the transformer and the fuses.

Finally see if there is an inline connector on the GI lines from the transformer to the fuse and make sure those pins don't look tarnished.

It would be good to check another pin of this era to see what the output on the GI line is there

Playfield GI is probably the non-fuse all the way to the left and the one to the right of it is backbox. Measure across the bottom of both of those

EDIT: Figure out which the 2 big yellow wires from the transformer go and measure across those 2

Is the transformer hi-tapped?

An easy way to check for high tap is measure voltage at the coils. It will be high too. Not definitive but a good clue if it is

If it's not hi-tapped I wonder about putting a high wattage resistor on the lines to bring the voltage down