As shared, the turret is one major identifying feature that separates the Leopard 2A0 through 2A4 from the 2A5 and 6. Here are some of the major differences and implications for a RC tank conversion. Note that Heng Long uses an entirely different internal layout, so these challenges are quite unique to Tamiya - yellow is the reference line, with red being the stock 2A6 internal layout and blue being the 2A4's revised layout. Green highlights the relocated radio receiver. Everything aft of the recoil gearbox had to be shifted forward, except for the receiver, which went into the notch next to the power switch. In this photo, I have used a 1350mAH 2S LiPo in order to fit within the reduced volume, as the consequence of shifting things forward was that the battery compartment suffered. Gotta do that, or else the tank is not going anywhere!

In this photo you can also see the obvious difference of an L/44 120mm gun as opposed to the L/55 on the 2A6.