Don't worry yourself about the quality of that steel. The
Argentine Mausers were THE most sought after action by Custom gunsmiths when such a thing still existed. Those actions were known to be smooth as glass, slick as spit and fast as lightning. As peace time built rifles they were lavish in both build quality and materials.
A true example of the gunmakers art. If I had laid eyes on that carbine it wouldn't have been in the rack for long.
I don't know where you read that but whoever wrote it has no idea what they're talking about. Argentines have been built with some of the most wicked magnums and custom wildcats on the planet and are plenty strong enough to deal with them.
The 7.65x53/54 cartridge is within the power range of the 308 and is a handy little round. I hunted with it for years with good success. It tends to be loaded light due to the weakness and limitations of the 91 action. I strongly suggest handloading this particular round and develop a load to take advantage of the strength and accuracy of these rifles.