Dusky,
That 30gal tank would add quite a bit, good point. If the drains are just at the waterline without that tank that's not bad at all given the weight increase with the 4s. I considered filling in more of the transom on mine so it would fit a 25" shaft but really did not like the look. So I opted to keep the 20" transom and added about 3" to either side for splash etc and still keep the over all lines of the original.
I had to show my daughter the pics of your boat on the water so she would get an idea of what my "project" might look like one day...she really likes the boat especially with the Suzuki on the back. It's all about lookin' good!