GullyFoylesbike

So instead of starting work this morning I went and double checked the numbers in my copy of Sutherland's.

For the stronglight spindle they list the bearing-bearing measurement as all 56mm. For sugino spindles you're looking at 51.5mm in the middle. So the sugino cups are together ~4.5mm thicker than the stronglight ones. You're looking at 1) the adjustable cup sitting 4.5mm further out and 2) you'll lose ~2.25mm of space for your crank arm on that side as well to the thicker cup.

My Gitane Grantour has a frozen Stronglight fixed cup, an italian length spindle, and a Sugino adjustable cup on it, and that works just fine. But that combo has the adjustable cup pulled ~3mm further in than what you're looking at doing. All in all I don't actually have a sure answer but maybe just eyeballing how much of your threading 4mm would be on a cup can point you in one direction or the other.