The 3.75 parts work fine but they don't really make a good Saturn V because the proportions are all off. Even using procedural fairings you only get close to the right look, even if you can get the right functionality. the only place the saturn v im working on now isn't right is at the S-IVB, because the parts are the wrong size starting right there. No big deal, but im sure we've all made a Saturn V and it'd be nice to do it right with stock parts. As for your second point, you can do that mission profile with 3.75m parts. I know because I just did.