Get the biggest thickest ( usually sold as high tensile ) washers that you can to go under the head.
I regularly make up my own .
This is to spread the load and prevent the bolt collapsing the hole onto the bolt

email (option): bsansw1@tpg.com.au

Very good advice...I forgot to mention that...Ian

email (option): ian@wright52.plus.com

What size are the washers? I am thinking grade 8 washers used for Diesel engines could be the ticket. Hopefully there is a close USA size.

email (option): cbhaws@verizon.net