If gearboxes are a concern, we could equip all cars with the close ratio transmission. Non adjustable so tuning doesn't come into play, but also wakes the cars up. In an enduro league i was in, we allowed close ratio trans, fixed suspension, racing clutch and flywheel but no adjustable parts. Worked out pretty well. You will never completely balance such different cars though. Thought we had it in pre season testing, but in a live race with the draft and fast tire wear, the cars separated themselves.
If you are really fast, challenge yourself with a lesser car. If you are not fast, pick one of the top cars. It is up to the racers to be honest and fair to complete the balancing process.