Very mild 318 - Comp 252H vs 340 cam?

To check the rear gearing. Jack up the rear of the car and put on jack stands, put car in N and spin the drive shaft so one of the U joint caps in facing you and dot the cap with a paint pen. On one of the tires put a dot on the sidewall on the ground side. Now spin the tire and count how many times the drive shaft spins. This will give you a close guess as to the gearing..