I am going to look at a nice 1966 3.8 tomorrow, and am very interested, but I prefer the looks of the MKII rear wheel area. I like the more rounded opening of the MKII. Would it be difficult to modify the 3.8 to the MKII looks?
A few people have done it, Usually by just changing the shape of the lip, rather than implementing a spat like the Mark 2. From what I hear it isn’t particularly difficult, and it doesn’t compromise the structural integrity. I think it makes the car look better.
My plan was to do it, but my body man refused. He also wouldn’t install the folding roof which I installed. He’s a great body man, but a bit of a stickler about some things.
While the car looks like it would have been a great restoration project (good car with very minimal if any rust, with a spare parts car for $5,000 total), I have decided to pass…If it had been a MKII, I would have been all over it. I have enough projects, and the 3.8 just would never be a MKII; no matter how much I modify it.
The S has a more trunk space, and the IRS. It isn’t as quick as the Mark 2, and the shape isn’t as sweet as the Mark 2, but it is a better tourer.
I found this pic , you can buy new MK2 inner rear wheel arches , that would be a good starting point , remove the back wing , weld in the MK2 inner wheel arch then cut the rear wing to fit , job done lol 
As an 'S owner this does not look right, I’ll be replacing the rotten areas with the correct shape when the son-in-law and I get into it.