The cat photo attached is a sled I built , it has a Polaris Star skid , 121 modern track , and a panther tunnel that was shorten and boxed after the install. I will get better pictures of the skid if anyone is interested. The slide rails are modified ( home built big wheel kit concept) with Cat 9” rear wheels and axle .
Remember your mounting location can only be transferred when the front drivers are the same size.
It was interesting on this project I did , how much of the track length was absorbed with larger drivers vs the Star sled, and adding the larger rear wheels.
The sled was built to oval , the skid was modified to look and attack like the mid 70’s Z skid. The 15” skid required the wider axles to fit the wide tunnel, and tube spacers are added to center the skid .
As others have said , pre-planning is key , if all wheel size remains the same it is a simple exchange, hole location from front axle back to mounting holes , also height of hole location from running board TOP edge Not bottom edge .
Tunnels vary in thickness and track clearance is from mounting hole to top of tunnel .
Best of luck ! Can’t wait to see the results !