Use the pressure rated on the door, not the tire. The doors number is based on the weight and weight distribution of the car, which effect your tire pressure.
I assume you are talking about radial tires on your classic Stang? If so, I think you are gonna need more psi that what the car's bias ply tire specs originally called for. I recently installed BFG's on my '68 and at 30psi they are sagging and bulging as if underinflated. I increased it to 35 psi , the car rides and handles better and the sagging is now just the normal radial tire sag. Hope this helps some.
th etire pressure to use is the one that makes the tread flat on the road with the weight of the car on the tires. on modern radials that is usually between 30 and 35psi. though that does depend on the car as well. heavier cars, or smaller tires, need more air pressure.
Radial tires always look a bit underflated... use the recommendation on the tires, just be sure to check the pressure when the tires are cold. If you are carrying a load you can over-inflate a few psi. If you're using a wider tire than is specified for the wheel, you will have a greater sag than normal.
my guess would be that the weight of the car is resting on that much LESS surface area on small tires, thereby requiring higher tire pressures to hold it up properly.