There's a reason they stopped making straight 8s years ago you know :)

the longer the cranks, the more they can flex.

If you run 2 3 cylinder motors inline, you don't get the same result as a regular inline 6 - the crank journal clocking won't be the same, and instead of the 1 5 3 6 2 4 firing order (which evenly spaces things front to rear, and keeps it somewhat of a smooth running motor, no secondary imbalance) - you'll have 2 3 cylinders one after the other, and they are both inherently unbalanced motors.

It's not quite the same thing, but it's also why most v8s (outside of race only stuff) have 90 degree cranks and not 180 degree cranks (the 180 cranks, or 'flat' cranks make each bank of cylinders fire like a 4 cylinder engine, so better exhaust tuning to be had vs 90 deg ones, but they have a big secondary imbalance issue then. so pretty much all production v8s use 90 degree crank phasing. It means the firing order is uneven - they don't alternate from one bank or row of cylinders to the other - they instead go left right left left right left right right, and repeat - so you get 2 exhaust pulses in a row from one bank, then nothing for a little while. THis '2 in a row' exhaust pulses is actually what gives production v8s their distinctive thumping sound . Flat cranked v8s sound a lot different.

It's not quite the same thing, but 2 3cyl motors inline is asking for a rough vibrating driving experience. With very little benefit.

TO keep it datto related - despite there being some great inline 6s out there from nissan, you really need to look at a v6, which is shorter and weighs less for the same capacity as an inline 6 (perhaps a vg30 rather than a rb30 for instance). Yes the v6s have harmonics and vibration issues, but they are pretty well taken care of with balance shafts (in other words I'm saying look only at v6s that have balance shafts in their design,