There's verious free meshes out there with the German-designed 120mm gun. I think it's a little excessive for this particular tank. I doubt the turret ring is big enough for the recoil forces. So, are there any free models out there which have the British-designed 105mm? There's the M60, the Merkava, the Leopard I and late-model Centurions.

There are some 3d models of tanks mounting the Britsh-designed L7 105mm:

• Olifant, a South African modification of the Centurion. One is available at ShareCG.
• M60 - two are available at model.3dcool.net. Note that most models on this site aren't properly labelled (unless you count "Tank" as a particulary informative label), and the thumbnails on the menu are indistinct - meaning you have go to each one's page to identify it from the picture.
• Model.3dcool.net also has a Merkava 1, and possibly some more 105mm tanks.

Other tanks carrying the British 105mm (which I don't know of any models for) include:

• Modified Israeli M48s (called MaGachs)
• CM11 Brave Tiger - a South Korean tank with a M48 turret and M60A3 hull
• Modified M47s in some upgrades (for example, Spanish M47E1 and M47E2)
• M1 Abrams
• EE-T1 Osorio
• K1 Type 88
• Merkava II (as well as the Merkava I)
• OF-40
• Pz-61 & Pz-68
• Stridsvagn 103
• Stryker MGS (Mobile Gun System) armoured car. For some time, the MGS overturned (when the gun was pointed to the side) or sheared off at the axels when the 105mm gun fired; I don't know if that problem has been successfully fixed.
• T-54 in several upgraded variants (for example, Israeli Tiran-4Sh)
• T-55 in several upgraded variants (for example, Egyptian modified T-55s)
• TAM medium tank
• Type 74
• Vijayanta
• Ramses II
• Centauro 8x8 armoured car

Incidentally, I don't think the problem with the size of the turret ring, compared to gun size, is to do with the recoil forces - the problem is usually that the turret ring is too small to accept the size of a weapon.
However, considering that the Israelis upgunned smaller M4 Shermans to carry modified French 105mm cannons, I wouldn't say that this tank couldn't be upgunned as well.