Well, it did take some modding (mainly cutting out some material from both ends of the W123 dashboard to get it to fit the /8's firewall and what not) but somehow they got it to work and appear to be stock! The only issue was that the /8's original welded-metal steering column would look so unsightly jutting out of the gaping hole where the W123's steering neck would fit. Other than that, not much in the way of modifying the electrical connections was needed, and the oil line to the oil pressure gauge was a direct fit, AFAIK
That said, my confusion stems from the fact that my uncle, as was (and still is) customary in the country, had imported his /8 out of Germany sometime in the late 1970s, and the car already had the W123 dashboard fitted
when he got it! So I find it rather surprising that someone had already thought of such a mod so early in the car's (and the W123's) life!
Also back in the 1990s when the supply of used parts for the thousands upon thousands of w110s that roamed Jordanian streets started to dwindle, and good, crack-free dashboards became harder and harder to find, many people swapped out the dashboards of 190s for /8 dashboards; all it took was to cut a little material our of the right and left corners to get a near-perfect fit. I know this because for some time in 1993-94, I used to own a 1964 190 with a /8 dashboard that had been swapped in by a previous owner, complete with a center console and floor-mounted shifter! Interestingly, very little (if any) modification of the electrical connections was needed to make everything work (again, the oil line to the oil pressure gauge was a direct fit, IIRC.)
Another popular mod to the 190 was swapping out the M121 for an M115 engine/transmission, which was a pretty straightforward swap with almost zero modification needed (the M115 bolted right on to the 190's original engine mounts!) I even knew a few guys who swapped out the rear swing axles of their 190s for a complete /8/W123 rear suspension assembly (which did take some considerable modding to the bodywork, but nevertheless seemed to work somehow!