A photo won't help much; you need dimensions.
Without exact dimensions, the easiest way to get the holes right is to transfer their positions to a paper template. Simply trace the positions of the nut plates on your amp chassis to a large sheet of paper, then mark and drill the case. Just be sure that the chassis template is in the correct position before you drill.
The screws are 10-24 x 1-1/4-inch Phillips oval head machine screws. Washers are called cup or finishing washers. You'll need only four of each.
Regards,