I fabricated a metal clip that hooks onto the bottom of the dash where the fuse box normally mounts with a screw.
Now I can unhook it and hold it in my hand…..much better access but only what the slack on the wire allows
This is good advice. When you check continuity, put the tester probe at the top of the pins as the solder may be broken at the base of the pin , causing intermittent continuity.
If it’s broken , simply re-solder.
I installed a top manufactured by a company called Kee on my 68 gtx years ago. I believe I bought it from legendary. They are a supplier to many outlets. I did fit very well , but I can say it’s not easy to install without those wrinkles you’re talking about.
It might be an illusion but the wheel cylinder appears to be sagging down. Are the mounting holes for it in the backing plate enlarged or “slotted “ ?
As already said here , the brake shoe slot for the wheel cylinder pin could be off.