No documentation would be needed in the US. In 1975 the only common wheel diameters for cars were 13" , 14" , and 15". Anything larger in diameter would have been for a very old car or a truck. Wider 15" (and 14" and 13") tires and wheels were readily available. You just bought them and put them on. No one had to approve a different size. It is still like this, but today the common sizes are 16" and larger.