What if the Italians were equipped like the Germans in ww2.

You could, but why would you want to? AFAIK, the Archer was the only tracked SPG to do so. Although, there were some wheeled SPGs that did so.
http://vignette3.wikia.nocookie.net/world-war-2/images/5/50/Portee2PDR.jpg]/img]


[QUOTE="knightdepaix, post: 806288, member: 30666"]2) could a backward driven Italian tank chasis mount a larger caliber and heavier gun ? An Italian Nashhorn ...[/QUOTE]
It has nothing to do with which way the gun is facing, but how much weight the tank chassis and suspension can effectively support. If the chassis and suspension cannot support the additional weight...It matters not if the gun is facing fore or aft.


[QUOTE="knightdepaix, post: 806288, member: 30666"]3) If possible, how about an enclosed in casemate? An Italian Jagdpanther...[/QUOTE]
The Italians did not have a tank chassis capable of taking the weight. They might have developed a Stug IV knockoff, based on the P 26/40 chassis, but production would have been very low, if any, before the Italians surrendered in 1943.

 

You actually have something like an Italian Nashorn in the 90mm SP guns that were meant to to provide some long range protection against T34 and KV for the ARMIR but ended up in Sicily where very lightly armoured restricted traverse AT SPs were not particularly useful. The big point is the Germans used a 22t chassis for the Nashorn, the Italians a 13t one.

 

The German had the idea of combining Pz3 and Pz4 into Panzerkampfwagen III/IV and VK20 series was a design. Am I right? If so, how about did Axis allies borrow these ideas to their own tank developments. If Italian tanks were interior, unsuccessful German design can still help?