Corrected the pinout listing above. Video and GND were swapped. Removed the video pin text as we're not interested in that for this test. Any GND pin can be used for testing purposes. They are all connected together.
I just took a resistance reading from +5 pin to GND on a fully working Space Invaders Deluxe CPU board and surprise! this one read 5K+ ohms. So I would say if you are touching the right pins with your meter then you have something leaking to ground on your board.
What you do to narrow down the issue is remove the 8080 cpu and take a reading while it is out. If the reading is still low then do the same with the eproms/roms. It's normal for the reading to get a little better as you remove and retest, but you will be looking for a significant difference. If it doesn't improve then unsolder one leg of the tantalum caps (tear drop shaped) in line with the rams. Do one tantalum at a time and retest until you find the bad one or replace them all at once if the problem is there.