I've got a 3FE powered 80 with an H150F behind it. It's a 275mm clutch I believe, and yes 10 spline for the 3F/E and 21 spline for the 1HZ. I believe the 1HZ was 275mm clutch as well. My research seemed to indicate that the input shafts were all the same length and OD. Thus, as far as I know...