I got my 351C rear cover off ebay (
www.ebay.com ). It ran something like $20.00.
I got my bolts from
www.fastenal.com. They are grade 8 and work great.
If you are asking about the three bottom bolts, in the US, they are 1/4 x 20 x 1/2 ( use 3/4" and a washer )
Hope this helps.