@Snap615 Yeah all I was doing was pointing out the failing points if this were to be real. If it was an SPG it would probably have to have the turret scrapped and have a new casemate or larger turret built around the gun.

@DeezDucks Thx a lot for telling about the reasons. Knew that 105mm was loaded on AMX-13, but did not knew why there were no 120mm version :P (Actually, I've never thought about it)

And I'm just thinking of the possibility of make this to SPG...

I can give you many real life reasons why this is not fixed.

1). This is a LIGHT tank, that means it's meant for scouting. You do not need a behemoth 120mm gun.
2). The 120mm gun is very heavy, there is no room inside the already very cramped auto-loading turret to add more counterweight. Therefor this tank would always have a depressed gun
3). Loading shell's the weight of 3 small children each is very hard for the auto-loading system in this WWII prototype. Unless you're okay with 5 minute long reloads I don't find this very viable