I see lots of engines strapped (and not strapped )on top of an old tire. A straight six on its side (assuming any accessories are removed from to bottom side) would ride nicely that way.
I use a tire also. But i usually lean it and sometimes against a tire. But regardless, always strap it. Better with 2 straps. They will move, bounce, rub, fall, and cause a big disappointment when least expected.