There are a lot of different alloys of aluminum out there whats the best choice for the 1/2 inch angle aluminum for offshore mounts?

Every time I've done any engine mounts I've used the 6061. Anytime something needed formed like dash panels and such I used 5052. The 6061 is going to be the superior alloy for strength. That's what I use for all my engine mount angles for what it's worth. Good luck. Best (cheapest) place I've found is onlinemetals.com

Most are 3/8, I did my last ones in 1/2 and anyone that saw them said holy sh!t those are huge lol