Just for giggles, since I'm a nerd:
That looks like you'll be using about a 7/16" diameter bolt,
grade 5 bolts are rated at 150,000psi (tensile)
grade 8 bolts are rated at 180,000psi (tensile)
Maximum allowable shear stress is
approximately half the tensile strengh.
G5max = 75,000 psi shear
G8max = 90,000 psi shear
Area of 7/16 bolt is (pi)*(.21875^2)= 0.150 sq. in.
Maximum Shear Force allowed on a 7/16 G5 bolt is
11,250 Lbs. Now if you assume that the load you're carrying is evenly distributed in the bed, and you have five G5 bolts on each side, for a total of ten - You THEORETICALLY should be able to carry 112,500 Lbs in the back of your stovebolt.

(just based on the spring hanger, which is probably not the weakest link).
Of course we also know that bolts usually do not fail in pure shear, just as they don't fail in pure tension, or torque - but usually a combination of all three! That's why we want to use a factor of safety of about 6...
All this nonsense to say - hey, use those grade five bolts and feel confident!
Bluedawg
BTW, a fine thread fastener is stronger than a coarse thread fastener, so if you have any doubt, go with the fine thread.