Myself I would rebiuld it, but the 4.3 is a good motor, you should be able to find a decent one at the parts yard. If your having someone else do it, it would be cheaper to replace than rebuild. However a rediuld will almost always come with a better warranty, its hard to find a used motor with more than 30 or 90 day warranty 90 days being about the limit. If this Blazer is a keeper, and your mechanic has served you well in the past, then I would go a rebuild. With a rebuild you know what you have.