What is the factory tire size for my 69 GMC

[69 GMC LWB Pickup]

I have a 1969 GMC 1/2 ton truck 2 wheel drive. I want to get new wheels and tires and don't know what the factory size is. The truck has two different sizes on it now and i don't know which one to use. It has a 235/75/15 and a 235/70/15. That is almost a full inch difference in the overall diameter. Can anyone tell me what the factory size is supposed to be so i can choose the right wheel and tire. This is important because i want the speedo to be as accurate as possible. Thank you to anyone that responds.

[special-K]

There never were radials or metric sizes offered on these rucks,so you have to go with the closest radial/metric. There were also various options from factory in the bias ply. The 75 series is the closest match to the original 78-series profile,so I like the 235/75s best. The standard size was a bit smaller,like 225/75s.