Following your advice, I removed the skirt on my Italeri Sherman.
It does look better and simpler...
However I have a fitting problem : 1 mm gap on the side, but mostly a good 5mm gap on the fenders at the front, between the fenders and this middle round piece...
I'm looking at some reference pictures, and it looks like there is actualy a riveted plate on the joint on the side of the tank, but none of that in the kit...

What is the best way to correct do you think? I think I can easily fill the gap on the sides with the putty.
But at the front, between the fenders, there is quite a big gap. I'm wondering if I should put some plate there and cover with putty...

Any idea?

I've pulled out my Italeri Sherman to see if I can spot the problem, but I don't. Are you talking about where the fenders join the transmission housing? If there's a 5mm gap there that's pretty big. Can you squeeze them into place, and super glue them? I find that if a part is a bit warped that's easier than trying to use liquid cement.

Can you point out here what you're talking about?

Wow 18Bravo, that's a sweet Sherman you got there!
Yes, the big gap is where the fenders join the transmission cover, and I will try the superglue. There will still be a gap after that, but it will be easier to apply the putty. Not sure about the superglue because there is quite a lot of elasticity in the part, hopefuly it will hold...

The other bit I'm asking about, is not showing on your picture. It is on the sides of the tank,where the side make a 90° angle above teh tracks. That's where the skits used to be glued. Now it is just showing a small gap that I could easily putty. But I'm wondering why on the reference pictures I can see a riveted plate, maybe it is another model of Sherman?

I believe you're talking about the mounting plate for the sand shields. Some photo-etch comes with this, but it's a waste of money in my opinion. I use strips of .010x.040 styrene and drill #80 holes in them. Add evenly spaced welds along the top with either super glue or two part epoxy.
Hopefully they show up here:

Sounds like your upper hull isn't seated far enough down. The upper hull should fit outside the sponson bottoms (the ledges at 90degrees to the lower hull sides) so the gap is only visible from below. This might also explain the gap at the front of the tank.

Try a bit of trimming and test-fitting until the fit looks better before gluing.

It's glued already! :-) :-) :-)
I did not pay too much attention to it because I used first mounted it with the skirts/sand shields on. Now I took the skirts off, it shows.
But I will putty and sand, should be fine.

I have also looked at some more reference pictures, and it looks like some tanks never really had mounting plates on, so I will not bother, even if it would add some fine details.

Thanks for your advice!