This is what I found:

"This is what happens to all system 11B and 11C games (with the Aux driver board) when you install LED flashlamps into solenoid spots 3,4, 6 and 7.
There are tie-back diodes on the large TIP36 transistors that connect those circuits to the 50V power source. There is a some feedback into these circuits from the flippers that is not apparent until you install LED's.
To fix this, Cut diodes D3, D4, D6 and D7 from the aux driver board. Then add diodes onto the coils for solenoids 3,4,6 and 7 (banded end of the diode goes to the power wire)"

"I believe any of the 1N400X will work. I usually use the 1n4007's"

https://pinside.com/pinball/forum/topic/ww-tech-help-flippers-trigger-flashers

Seems pretty straight forward.

Did you remove the warming resistors as well?

As long as the warming resistors remain in place you will have errant "flashes". Step one in installing LED flashers is to remove the warming resistors from the lamp boards.

Derp, sorry brain malfunction. You are correct. Thus the diodes....

Great, to hear. I have to do this to my SoF. I placed an order from Great Planes last week and am still waiting for the bits. I am glad it worked out for you. I wonder why Herg is still having issues on his.