I like the idea of a shortened M76. I shouldn't think this will give you any issues with the gas system, but that would be a question for your gunsmith. Add correct optics for closer range work, and you should have a good rifle for your purpose. There's not much a 196 gr 7,92mm bullet won't stop, and the shorter barrel won't give you much of a velocity handicap. I was very impressed with the rifle when I fielded it in Bosnia and Croatia many years ago under less than ideal circumstances. Accuracy with military ball and commercial match ammunition was outstanding. Our guys preferred it to the STG 58 and G3 rifles they had available for counter sniper operations.
Wish I had purchased one here in the 'states when they were imported some years ago at an affordable price. Ammunition availability was the sticking point, as most of the military ammunition imported was corrosive primed and the rifles are very hard on brass making reloading a difficult proposition.
Another rifle I just thought of that should be available in your part of the world is an Austro-Hungarian M95 Mannlicher. Not sure the availability of the 8x56R round in suitable sporting loadings. Another option would be a Swiss K-11 or K-31 if those are there. If you can skip the bayonet, a sporting Blaser or Steyr straight pull would be a fine option. Hard to beat those for fast shooting in close.