Yes, speed rating of the bearings would be my biggest issue with them. Besides, bearings aren't expensive. I just bought a couple for my mini-bike for only $4. However, I like to encourage doing things different than the way people recommend, just to prove it can be done. That being said, here is a set of plans with 1" holes for your shafts and bearings.
If you want to go really cheap on an 8" wheel, Northern Tools has 8" urethane covers cast iron casters(orange) that can be balanced rather easily. Replace the bearings for about $6 and you have a rather nice 8" wheel. Just weld it to a piece of 1.5" square stock. I've been using mine for a few months and it isn't bad.