Windage Tray on 273 Question

I'm building a Commando spec 1967 Commando 273. We decided to install a windage tray. I located a tray and hardware.
I understand that 318 and 273 engines used washers under main bolts to add spacing for the tray.
I measured the bolt length as well as the depth of the main bolt holes. Although the bolts are longer than standard, there is enough depth for them and I'm able to torque them to 85 ft lbs. However, the windage tray will interfere with installation of the oil pan.
Looks like I will need washers to give extra height. I assume that these washers should be hardened. Any suggestions as to where I can source some washer?
Thanks in advance.
Kevin