Grade 8 bolts are measured in tensile strength I believe......how far they can stretch when tightened down before they break. The hitch pins are rated in shear strength. I don't think you can compare the two.
I'd go with hitch pins. Yes, the tensile strength rating is more on a G8 bolt. BUT...you're tightening down that bolt. It has forces on it in multiple direction. You're stretching the bolt and side-loading it. Shear and tension forces simultaneously. Plus, the clamping tension gives the side-loading forces more leverage. The hitch pin just floats in the tongue so it only gets loaded with one force.
For the bolt to overcome the 2-force problem, you'd have to crank it down tight enough to squeeze the two parts of the tongue together to create enough friction to overcome the forces the truck is putting on it when starting/stopping. That would remove the side-load. But I don't think you can possibly crank that bolt tight enough to do that.
A grade 8 that is too long, so that the non-threaded part is longer than the alignment holes in the tongue, may be the way to go if you're dead set on a bolt. That way it doesn't put any clamping forces on the tongue or stretch the bolt. In that case, it's basically just a G8 pin.