I used ACE carriage bolts when I mounted a 54 235 in my 40 last summer. While my brother pushed down so the square holes (not an oxymoron) caught the carriage head I tighted from below. I didn't use a torque wrench so I couldn't tell you how tight.
Most of the weight is resting on the frame so the bolts don't carry much load. If you flip your truck then it might be a problem, but then you'll have other worries.
