New Bucket Seats

Well I removed the old buckets seats today, I will need to re-drill only 2 new holes per seat for new frames to fit the floor of my 1972 Duster....
I would like to install new hardware, bolts, washers, and nuts....
I need to know what type of bolt....Allen Head preferred...
What size do you think I will need....
I read a few say 3/8 Grade 8....
Also would you install Fender washers on the bottom side for support.