We used to call this "kitbashing" in the old days. Some models match wheelbase with existing chassis. There are some adjustable chassis too. I like the Artin chassis because they are cheap and have two wheelbase settings, but there are brass chassis that are much more adjustable as well. You can make a simple styrene chassis too, depending on how much power you want to apply.
I also posted this on another thread, but you can use the actual model chassis as a starting point in some cases:

That's a Monogram Ferrari F-40 snap tite kit with brass tube for axle bushings and a Scalex guide flag.
So by all means, choose your favorite kits and bash away!