Great work Carlos!
Your Sherman with the smaller gun is something I have been thinking about for some time. Though, my version would keep the Sherman turret and install a 57mm/6-pdr to create an infantry support tank. Something that could have been assigned to the tank company of the infantry regiment.
Your M18 with the M8 Turret also looks like a great combination of parts. I see no reason why you could not increase the size of the main gun provided there is room for the weapon to recoil in to the confines of the turret.
Sherman turret on the M18 looks just as I had imagined it would. Wonder if there would be sufficient clearance under the turret basket for the thing to fit as a drop-in modification. Ammunition storage might be a bit less but it would be a great little tank with a speed advantage.