I'm getting ready to make a batch of rudders for gas boats and I don't use anything BUT 7075 aluminum.
I can't tell you how many times I've seen a fast hydro or rigger come in with a bent rudder because someone used the wrong material.
7075 costs about 2 1/2 times what 6061 does, but it machines almost the same.
I'm also going to make my version of a drop down engine mount.
I made some mounts about 10 years ago from aluminum bar stock and with my mounts, I can get the engine so low in the boat you can't slide a quarter under it.
I had to make the mount wider to do what I wanted it to do, so my mounts will be 4 inches wide instead of 3.5 and will use 1/2 inch rubber isolators instead of 3/4.
I'm making 30 mounts. If they work like I think they will and I sell out quick, I'll make more. If not, I guess I'll have mounts for lots of new boats.