Hi Oliver
Don't worry about the tyres as the available list is the same between Class 4 and Class 5 so changing is no issue (as long as whatever you choose is from List 1A or 1B. For power, you can definitely change depending on the Dyno results - if its 220bhp or less, that would drop you down to Class 4. If more than 220, then you'd remain in Class 5.
For numbering, I'll put you in Class 5 for now. If you are in Class 4, that would change your competition number as we try and keep the numbers associated to classes, so we can run on the day in class order.
Do you know when you'll have the results? No issue, I'll just make the adjustments in the spreadsheet to put you in the right place.