You can easily fill 28-29 KG in your cylinder, please note that around 3 KG will remain in your cylinder due to cold weather right now and you will be able to use around 25 KG, you will get an average of around 25-30 Kms per KG if you car is tuned. Say if you will get atleast 25Kms per KG then you will be easily able to reach 635 Kms, you can get your car refilled from Rawalpindi aswell. Please bear in mind that you will need to get hot water pipes connection installed within your kit which will circulate the coolant of your car in the kit for maintaining the temperature.
There will no problem in driving for 8 hours, just make sure you get water connection and gas setting in good manner. Kindly get hot water pipes connection some days earlier so that water leakage could be checked
Happy Journey!