I do not understand why some think a grade 8 bolt is necessary. The proof strength on a grade 5 1/4" bolt is 4165 lbs of load. Times that by 4 and you have over 16,000 lbs of lifting capacity.
As overedriv stated, the weakest point is the quality of threads in the manifold. Grade 8 bolts make no difference with this factor.
I personally use an engine load leveller. Attaches at 4 corners, its longitudinal angle is adjustable so you can tilt the engine as it raises or lowers .
You can buy them at Autozone, Oreilly's or similiar for around $30-40.