Motorhead here. As I understand it, Lancia started with a narrow-angle V-4, progressing in various models from 10° to 20°, finally replaced with a pancake (flat / 180° opposed) arrangement (like a modern Subaru or the original Honda Goldwings). But pancake engines have packaging issues.

The V-6 was to sort out the vibration problems the narrow V-4s had. That was a 60° engine, midway between 45 (tertiary vibration naturally cancelled) and 90 (secondary vibration naturally cancelled). With either of these, it's possible to put a bobweight on the crank to smooth things out a bit.

They all looked to favor a relatively longer stroke than bore, making for higher torque at a particular RPM, but I think they did this as the Italians didn't have good spring steel available at the time (same reason Ducati came up with their 'desmodromic' valvetrain in order to have a higher 'redline' (major PITA to adjust). As it is, The V-4 was either single or double OHC, the V6 was OHV (so pushrods; I see nothing about high-underhead cams, so probably looong pushrods with plenty of inertia). I've no idea if the V-4 DOHC had rocker arms or was cams over buckets, but it probably had a higher redline than the V-6.

Nice that you'd write something engine-related here, but next time (and I hope there's a next time) maybe a little more detail on why a particular engine might be of interest - although I suspect I'm an audience of one here.