Choose a place that has plenty of parking and easy access to roads going the way you want. Access to restrooms would be very desirable and maybe food and drink depending on the time of day.
Plan on roads based on the type of cars in the tour, don't pick Interstates if you have Model A's. Stay away from gravel and the dust it creates.
Most people would like a stop after an hour or so, maybe sooner if the roads are rough. A stop with a restroom would be good.
Try to keep to the speed limit, but base your speed on the quality of the road and type of cars in the tour. Keep enough space between you and the car ahead of you so you can always stop safely.
When on busy roads allow enough space for others to pass safely.
Except for the leader older cars should be up front, newer cars to the rear and modern cars at the end. Older cars will be the slowest, best to keep them up front so they don't get left behind.
Find a place where everyone can park together, best if the lead car goes to the nearest point in the parking area so it can be the fist car out.
Decide in advance if the drive back home will be in a caravan or individuals travel at will.