904 18 spline input shaft strength

Hi all.
I have a rebuilt 904 which has the early 18 spline input shaft. What sort of power is the smaller shaft good for. Wondering if it’s worth the effort to change to the later 27 spline? 318 that makes roughly 400 HP.
Street cruiser. Will the 18 spline input shaft be the weakest link?

Or should I look at a 727. 727 will last but takes power and if not built properly will be a bit of a slug.