@cbarge would be the expert here, but from memory, I'd say that 235/75/14 would be a perfect "stock" size. It's likely your speedometer has been reading fast with the size you currently have on your car, as the diameter is too small, and makes the speedo read faster than you're actually going.
Keep in mind that 14" tires with the narrow whitewall that dresses up our 65-68 cars so nicely is very difficult to find in 14" sizes any more, and if you're going to be buying new wheels, I'd respectfully suggest you move up to 15" wheels, as this will give you more options for appropriate tires. As well, if you were ever considering upgrading to disc brakes, you will need a 15" wheel to clear most caliper setups.
Your car would pop with 15" chromie magnum 500s and 235/75/15 Hankook Optimo tires. This is what I run on my 66 Monaco. You can go considerably wider, but not too much taller. Too tall or too short a tire will result in inaccurate speedo readings. I believe you need a 28" tall tire... There is a tire height calculator out there that can help you figure that out, but again, don't go too wide. Below is the tire calculator:
Tire Size Calculator
Additionally, using that calculator, here are two "fat" tires that wind up being 28" tall - the first is 9.5 inches wide, and likely won't rub, but the other is 1.5" wider, and will likely rub on cornering etc. You might be able to get away with them on the rear, but not on the front.
235/70R15 28" tall, 9.3" wide, 15 x 6-8" rim is best
275/60R15 28" tall, 10.8" wide, 15 x 7.5-9.5" rim is best
Others will be able to chime in here with their experience, but keep in mind that each front fender and rear 1/4 from years other than 65 or 66 will have different dimensions and therefore may not apply in what will fit.
Also, if you have to change a tire, fat tires in the rear can be a real pain to get off the car due to the low wheel opening arch.
