I use grade 8 course thread 3/8" bolts for the upper bellhousing area. The same size that is already tapped there. Make sure you use a good solid steel spacer under the bolt head and not just a couple of cheap washers. I have seen the heads get pulled into the hole the bolt's go through on some stands if there wasn't a good support under the bolt head. Most stands use holes large enough for a 7/16" bolt to go through hence the need for a space. Also Make sure the bolt threads all the way into the block and still gets tight. I cut the threads to be the just right and use a grade 8 washer between the head and spacer to prevent it from bottoming out if necessary. A loose bolt or one that doesn't extend into the block enough is a problem waiting to happen.
For the lower bolts I get extra long grade 8 bolts that will fit through the holes in the tabs at the bottom of the block and use nuts and spacers to prevent a pull through. You can usually get away with a 7/16" bolt there.
Each stand is different so you may have to get a different length or dia. for your your stand than I would need.
Just the way I do it.
