When you put in the new boards, did you rebuild the plugs that attach to those boards? These plugs are one of the weakest points in these early Bally SS machines.

Does that power supply have a huge cap next to the transformer? and have you changed the lamp and sound boards? may not help, but sounds like the only things still original?
any switches loose or mangled from years of hammering?
bad diode across a switch or coil?

Looks like the only voltage way off is for high voltage / displays...which will not prevent boot,

As mentioned ALL the connectors MUST be rebuilt or you will be chasing ghosts in the machine till the End Of Days.

After that is done, power up while watching the MPU board and see what the diagnostic LED says.

The J4 connector is the only connector needed to boot the MPU. I have no idea on the Alltek boards though. I have only struggled through the original MPUs.

Congrats on keeping such a great classic title on route, it is a favorite in my collection.

My guess, the same as others, is that if two MPUs both keep resetting it is probably a bad/marginal connector.

I know they were rebuilt once, but if you can't find any other issue I would repeat the rebuild, perhaps a marginal connection at one of the crimps. I have had test points have the correct voltage, but that is only part of the story, a bad connection might not be letting enough amps through because of a marginal crimp or one where the wires have corrosion on them, thus the reset.

I know many think it is bad form, but I have soldered the molex connectors onto the wire after crimping them for critical areas like these, that way I am certain the wire to connector issue is solid. Also, I know if the wires are clean enough to wet with solder I know the wire was clean enough to have good contact with the connector. Some of the old wire in these harnesses has such bad corrosion for the first inch or two from the end of the wire that good conductivity is not possible without diligent scraping/sanding of the wire.

Best of luck

Regarding the extremely low voltage at test point 2 on the rectifier board: Can you compare that DC voltage to J3 Pin #5 on that board? Just want to make sure that test point 2 is ok. There is a resistor right before the test point that could be fried.

Assuming the voltage is legit (low), can you then check the DC voltage on your soldered wire connections E5 & E6 on the rottendog rectifier board? Each wire should be delivering ~100 volts DC from the transformer. Both of these will contribute to the Test Point 2 voltage. If these voltages are OK (and the connections are solid) then you might have a bad diode on the rectifier board. If voltages are Not OK at E5 & E6 then it's time to work upstream. Check transformer solder connections, then check the 115v AC pin connections (pins 6 and 7) on connector J2. Good Luck!