Consider this.
Rather than cutting extra spline length on the shaft - try modifying the hub of the clutch disc.
Tom Langdon suggested trimming the corners of the clutch disc hub splines. Take a die grinder and trim back the inside diameter corners of the clutch disc hub on the transmission side. That way you don’t weaken the input shaft at all. Trimming the corners of the disc hub won’t require as much detail work. You will need to remove any burrs with a file. I’m thinking a trim of 1/8” would be plenty.
Basically just chamfer them back? Is that what you’re describing?
