I’m a bit late but the trick to stabilize your hole saw is to nest 2 hole saws on your arbor. The big hole saw on the arbor first then a smaller hole saw to match the hole that’s in the car frame to act as the drill arbor. Go slow and once the big hole saw starts digging in it’ll cut correctly.

I had washers welded on my frame. The holes in the center of the washers are too small for my bushings to fit into. I bought a big step bit but it doesn’t seem to work.

just weld a bit of flat bar in the center drill a hole in it

Whats thickness is washers