TBH Ron I think you'd probably be better off sticking with the slide scanner... I have tried copying slides onto digital using a slide copying attachment which fits on the front of a normal lens. Like you I have had problems with cropping and it can be a bit tricky to get the right extension to keep within the available focus range. Even when I manage to get it all lined up I'm not sure the set up is capable of recording the very fine detail in the slide. Comparing the digitised version with the original projected does suggest that much of the finest detail visible when projected has not been captured by the camera.

Occasionally I have thought about getting some sort of back projection screen and copying the slides that way... IIRC that's what Julie Durn used to do to make duplicates of her slides.