If your going to build something custom, why not hard mount the alt and add a tensioner pulley (spring active auto or manual). Jeeps 4.0's have some really good solution to use as a template.
Not a bad idea. I just didn't mind busting out the static tensioner, and we already had a template from the factory location. It's easy to get to with the hood on, so tightening it occasionally won't be a problem.