I have the same boat as you. I run two beefy bags. I can get my speed down to 1.8 mph. You're going to enjoy your bags. Going into my sixth year with them and they are holding up good.
Check out Trax Tech tracks. I run two Big Jon Brutes that I slide into 16" tracks mounted on the transom. I also have gimbal mount tracks that hold my rod holders. Awesome quality products.
I agree with tbromund. I have two beefy bags that are going into their fourth season. Give Capt. Frank a call and he'll direct you to the proper size bag. I can't say enough about the quality of his product...........
Transom mounting plates are the way to go.You only need to drill two holes to attach it.This way if you make a mistake you are not putting extra holes in the transom.$10.00 bucks at Gander or your freindly marina...........