I'm pretty sure the top yoke for a 748 is the same as for 996S. Just checked the parts cat and it's the same part number.
Regarding changing steering geometry, a different top yoke CAN alter the geometry! It's a bit hard to explain but I'll do my best.
Imagine if you will a vertical line, that's the line down through the centre of the steering shaft. Now, if the offset between that line and the line down through the centre of the fork legs is identical for both yokes then the forks will be parallel to the steering shaft. Now, imagine altering the offset of only the top yoke. The forks are no longer parallel to the steering shaft.
So, if the offset on the top yoke is even marginally different than the offset of the bottom yoke, by the time you translate that over the length of the fork legs, it will have made quite a large difference to the trail.
This is what I suspect is going on in your case. It would also explain why the new top yoke was slightly tight to fit on as the holes in both the top and bottom yoke would not be in perfect alignment (axis wise).
Ducati use a similar principal to allow adjustment between 24.5 deg and 23.5 deg rake. The difference is instead of playing with yoke offsets, they adjust the overall angle of the steering shaft in relation to the headstock on the frame.
My advice would be take the top yoke off again and compare against the original. Bear in mind that any difference would only have to be very slight.
Cheers, Neil
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