First place to start is to look at what is on there now.
Many things have changed in 45 years and one of them is tire size. Just a guess, but you probably have 7.00-15 8 PR, but you need to check. you will want to note all the letters and numbers associated with the size - such as "LT" or "ST". You will also want to record the Ply Rating (PR) or the Load Range (LR), but I think LR predates 1969.
Better yet, take a series of photos and post them. We might be able to see something that you might not think is important.