Randy,
Yes, it definitely helps to have a lot of cars. I will check wheelbases of Slot.it cars to see what might match up to a particular car and see how the body mounts might work. The Slot.it HRS chassis have adjustable wheelbases and will fit a number of cars, but I don’t use them much. They are bulkier and heavier. With any of these conversions, body mounts will have to be re-configured. The Fly Ferrari GTO turned out to work well. Other cars may not.
Of course, nowadays there are so many 3D printed chassis available that will bolt right in. That would be the best bet for a conversion. If you have a particular car that you want to race, the handbuilt conversion may be the only option until someone comes out with a printed chassis.