I'm gonna say "nope" the later model 800cc bike front disc won't bolt up to a 88 750. That bike (like mine) should have a 4 bolt pattern for the front disc, the 800cc bikes have 6. I just received a nice package from Stefan Hessler today. New air filter, fork seals, left case gasket and a new front brake disc.
I'd be helpful if you posted a photo of your bike. That way we'd know to what you are referring....and it would keep us from looking at photos of all of us.