I am doing an LS1 conversion on my 9 bolt rear end. Will the axles seat properly with the GM LS1 backing plates ? They are thicker than the stock plates.

No they won't, you will have to make a spacer to makeup the difference.

Does anyone make this spacer ?

Not that I know of, I made mine out schedule 40 PVC.

Looking at the pic....2.550" is the overall diameter & 0.30" is the width?

S judging by the pic...It is just a 0.30" wide slice of a 2.55" aluminum pipe & the wall thickness is irrelevant? With a piece cut out just wide enough to slip over the axle housing?