A while back I decided to upgrade my alternator to a Delco 12SI.  I had to make a new mounting bracket for it since my original bracket was cast and couldn't be easily modified.  The install went reasonably well, after upgrading and repairing some wiring and everything was great.....  Until I turned my A/C on.  Then it screamed like a banshee.  I tried several times to get the stock tensioning bracket tight enough to keep the belts from squealing, but the best I managed was to get it to last about 2 days.  I eventually stripped out the upper mounting hole on the 12SI so I had to go to a longer bolt that would go all the way through and have enough room for a nut on the back side.  Still not enough tension.

So I took all the pieces of the top bracket off and started fabing up a prototype out of cardboard pieces.  Once I had something that looked like it would reasonably work I cut each piece out of metal one at a time and started welding them up.  Had to test fit things several times and re-make a couple of the cardboard templates when I saw how they'd fit together as metal.  In the end, I came up with a pretty nice system.  Here are a some pics of the finished product.  (Ignore the ugly welds.  I'm still learning the right incantations to tame my Harbor Freight MIG.)

The first view is of the assembly laying on it's side:


Second view is more of a true side view with it sitting on top of my Metric toolbox:


And last, here it is installed:


The bit that the piece of all-thread is attached to fits over the top mounting tab on the 12SI.  A bolt goes all the way through the stripped-out mounting tab and there is a lock-nut on the other end.  The all-thread fits through a loose hole in the other half of the bracket.  You control tension with the nut closest to the bracket, and lock it down with the other nut.  The silver nut on the end is there just as a (probably unnecessary) precaution in case the belt breaks.  Don't want the alternator bouncing around under the hood.

It works fantastically.  I now have a way to get the belt tight enough to stop the squealing plus I also have a very fine control over belt tension should I ever need that for any reason.   

good job. I'm going to be doing a similar project but I will be moving the alternator to the upper driverside of the motor. want it to be out of the water as much as possible. it will be mounted where the a/c unit is but I dont have the ac unit anyways. going to go with a gm alt also just not sure what one yet. plus it will be easier to get to unlike the stock location

Nice, I wonder why you couldn't get the belt tight enough? I reused my top bracket, just welded an extension on it so it was long enough. Been on there for almost 3 years now, no problems.

Probably because I had to go to a larger belt.  The pulley on the Alternator is a tiny bit larger than the old one too.  (Only size the local auto parts store had that would fit on the alt's shaft)