Metal stress/fatigue question

I'm looking to put together a setback bracket for an outboard motor. I need 16-18" of total set back. The motor weighs about 480lbs My current plan is to use 12"×4"×1/2" aluminum C channel as an offset bracket and then mount a 6" or 8" hydraulic jackplate to that. My question is 6061 or 5083 alloy. Or, does anyone have a better idea. Thanks!