Check your bolt buckets

I have looked at regular carriage bolts. Zinc plated ones have a much smaller head on them, galvanized cb's have a "little larger" head on them, and stainless steel ones have a head nearly the right size. SS ones run about $1.50+ each.
I will be using fiberglass bumpers, so I wanted to get as large a head as I could.
I may have to bite the bullet on new ones, though.
Thanks!
C