Driveline swap possible?

I'm nearing the end of a slant 6 to small block V-8 conversion project (I hope) and I'm running into all kinds of hurdles on this build. Can I re-use the original driveline from the 1970 - 904 w/ the newer 727 from 1971? Someone told me that all 904's are a 26 spline and that the 727 is a 30 spline. Any help would be greatly appreciated. Thanks