A standard 9 inch is a couple of pounds heavier than a 12 bolt (which is roughly the same as an 8.5" 10 bolt)... so I believe your thought toward an alloy third member will be the answer to your question.
As for the alloy third member... do not purchase a standard or light weight/duty case. It would be in your best interest to purchase a thru-bolt case from the get go. The difference in cost will more than make up for the durability and strength issue's.
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