I have not installed a 67 but can offer the following tips;
Alcohol provides a very slick lube that evaps. (We use isopropyl alch) for making electrical pin insertion in connectors for various systems on subs & carriers (NAVY).
I have a thin Cake icing metal spatula commandeered from the kitchen to sneak behind the triangle section to "PUSH" on the rear axel bumpers on a 68/69 car, which might be a similar insertion type-method. The spatula is not Sharp by any means so damage-cut to the rubber were not a worry or problem.
Jim