I've never had the need to install a connector to the transformer wires. When I rebuild these boards, I remove the transformer base from the machine. There are 4 - 6 black metal screws (depending on the game), then I remove the board from the 4 stand ups and then simply place the whole assembly on the bench and work on it - similar to the description on the Space Invaders restore.

I plan on writing a guide for the rebuild of the Bally / Stern Power Boards when I get time