I just mounted one on my pontoon boat. I had to play with it and adjust it a few times to get it right. It pretty much has to be perfect or the side imaging to work like it is supposed to. Nothing can be in the way L&R. Bubbles and turbulence will also interfere with it.
I agree with smooth move. I would probably mount it on the tab. Make a shim or custom bracket for it with vertical slots where you can adjust it up and down. It has to be even with the bottom of the boat maybe ever so slightly below even.
Once you get it mounted, if the engine is interfering you may need to tilt it up a notch when side scanning to the opposite side the transducer is mounted. But I don't think this will be a problem.
A buddy of mine just mounted one on his jon boat and rigged it up on a 2x4 somehow and just clamps it on the boat when he goes. He wanted it to be mobile where he could use it on two boats he has. Works great for him.
Just an FYI, if you break the mounting bracket like I did when I was trying to get mine adjusted, Academy has them on the shelf. BP and Cabelas don't. I bought an extra in case I ever break it again. Dang Plastic!
If it ain't yours don't mess with it!
Need a friendly place to talk hunting? www.talkhunting.com