Here's my 2 cents worth:
Any Stainless angle over 3" x 3" x 3/8" for outside, Inside I would use a full plate 1/4" thk across the transom to disburse the force on the transom. The plate should be a minimum of 4" larger than the distance across and above the outside area of the angle frame dimension. Bolts 5/16" dia minimum 2" down/inside from each end and then spaces about every 8" apart. Grind the extreme ends of the angle where they lay against the transom. All welds should be full penetration for the bracket and ground smooth on the side against the boat. This will not allow any brunt force against the fiberglass.
Do not use aluminum, you would need to beef up the thickness from 5/16 to 1/2" thick to compensate for strength and it would need to be out of billet aluminum.
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Tis better to be quiet and thought a fool than open your mouth and prove it!!
1991 V-20 cuddy I/O 350 volvo duo prop, 1998 15ft Grumman 9.9 Johnson 
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