23 spline v 18 spline

hello, i have a 4 speed that needs rebuild im guessing syncros and bearings,i have a 727 im going to fit in meantime just to keep me moving, my input shaft 18 spline i often wondered if this was just the input shaft that was "beefy" and i have read here quote::study: The A-833 four-speed was built by New Process. produced from 1964 until 1987, it is most noted for its strength and durability. From 1964 until 1972, it was built in two configurations-a 23-spline input shaft and an 18-spline input shaft. These shafts are not interchangeable alone, and the entire gearset is matched and must be changed as a complete assembly.......... is this true?