I can answer part of your question. I used 4 1/4" machine screws with a washer and split (lock) washer and a 1/4" nut.
I used 5/16 X 1 1/4 lag screws to secure my castors
The dimensions of the bolt configuration is very close to 3" center to center in one direction and 4" center to center in the other direction. You might be able to find that kind information on the Home Depot web site.