I remember getting 50mpg out of my I-mark, but usually it was closer to 40 because I drove mostly city and it was floored a lot as there was not much acceleration, everything was passing you, unless the downhill was long enough. When you had a hill in front of you that meant I would be decelerating and shifting with an amazing loss of MPH very soon. I didn't have a M35 back then, but now that I have driven many they resemble each other in this respect. Certainly was a dependable engine. If the glow plugs burned out, no problem, just don't turn it off, it would idle my 8hr shift at work and be all warmed up for my 3 mi trip home. Sadly the trunk rotted out and I didn't like diesel fumes burning my eyes, so it got donated to the church auction.
When I bought it new back in '81 the salesman told me the engine would easily outlast the body. I thought at the time that was an odd sales pitch, but it was the truth.