I use lengths of appropriate studding to attach an engine to a stand, which saves a lot of faffing about with different length bolts and packing washers. You use the bell housing boltholes on most stands. They are are almost certainly M10, I'd undo one of the more accessible ones if I wanted to check.
The easiest way of getting the engine on the stand is to bolt the mounting head to the engine first, and slide that into the tube on the stand.
Buy a 4 legged stand; they are much more stable than a 3leg one.
1" angle is cheaper, easier to work, and makes attaching the shelves much easier. Pay attention to the proportions, the cart I made for my TIG and plasma cutter was far too tall for its width. It was so unsatisfactory that I recently scrapped it and bought one, which is folded sheet metal. £40 from Machine Mart if you're interested. I do understand why you want to make one as it is a good first design/build project.