Traveling how far should I go before stopping to let my engine cool?

If it is not overheating you never need to stop except to pee and get gas or muchies.

Just keep an eye on the temp gauge, but unless you have a deficiency in the cooling system or engine you should not have any concerns about overheating. There's so much air flowing through a radiator when cruising down a highway any sound engine will run all day without overheating.

If you plan on averaging 67 mph you will not have much time for stops! I think your 12 hours will be more like 14 hours.

If you can go 75 all the way without slowing down it will leave you an hour + for getting gas, etc.