The actual 120mm M58 tank gun used by the M103 was kinetically more powerful than the Rheinmetall 120mm gun. If you gave it advanced fire control and updated ammunition (such as sabot rounds), it could potentially be a better weapon than the Rheinmetall 120mm.
I'd take it that people wouldn't be too thrilled with a Russian-style autoloader for this one......
Nothing wrong with Russian-style autoloaders. There are a great many myths about Russian autoloaders which were promulgated more to ridicule them than were actually based on facts. The one about loading the loader's arm is a well-known but false one. Carousal loaders have problems with safety in case of penetration but they can be fixed if careful attention is paid to isolating individual ammunition lockers and using a pressurised water jacket.
How could a 70s-era tank gun autoloader be done right though?
There were several attempts at autoloaders in the 1950s, 60s and 70s. In the West, the most successful was more than likely those in the AMX-13 oscillating turrets. The M103 and the Conqueror heavy tanks had them but they weren't reliable. Apart from that, there were quite a few experimental vehicles. In the East, you have the T62, T64 and T72.
It wasn't until modern electronics come along that you see them becoming reliable enough. The need for them has also increased with the increasing calibre of tank guns. 120mm is about the physical limit for most loaders. The new proposed and developed but yet to be accepted into service 140mm guns are beyond what most human loaders can reliably load at a sustained rate in the tight confines of turrets.
The French again are leaders in this with their Leclerc MBT, in the West and the Japanese Type 90. With adopting an autoloader significant advantages flow on in the design of the vehicle, with the armoured volume being decreased considerably (no standing human loader means a smaller turret and hull height). It also brings disadvantages in the form of one less crew member for other duties, which increases the out-of-battle things the remaining crew have to do.