[tooski]

On my 68 350/350 both U joints were/are the same. 1310 series. I'd be surprised if the U joints were different. I doubt that they would be different from the factory. If they are different the yoke on the driveshaft would have had to be changed, along with a different trans yoke. I doubt that would have been done. If you can, do a rough measurement of the u joint width and cap diameter. Different series have markedly different dimensions.