The 6-cylinder engines of the 260/760 vintage were V-6's, not I-6's (a.k.a., straight 6). They were made by a joint consortium of Peugeot, Renault and Volvo (hence their name, PRV engines) -- I think their names (i.e., the former two) says it all, viz. their reliability. These engines were reknowned for their cam wear (oil circulation deficiencies), although if you can find an engine that still has a good cam (i.e., only ran cleanest oil, changed oil very frequently, e.g., every 1,000 miles or less), I suspect that modern synthetic oils using ~5W-30 or 0W-30 viscosity would deal with this very nicely.
Keep in mind that V-6 engines have inherent imbalance problems -- this is usually dealt with by counter-rotating balance shafts, and/or crankshafts (and cams and distributors) with irregular cylinder synchrony, not to mention vibration-dampening mounts.
An I-6, on the other hand (assuming that it's running right), has no first, second or third order vibrations -- in theory it can run perfectly still. I've had a Mercedes straight six that I could balance a quarter on (with a little filed edge, so it wouldn't roll :-) -- loved to show that to friends; and I'm hoping to eventually get my '75 164 (in restoration) to be able to do that someday, too.