904 18 spline input shaft strength

If you have a "good" converter to go with the early 18 spline, I would stay with that. If you need to get a converter, then, convert the trans to 27 spline, and get a 27 spline converter.
Forget the 727

EDIT: What size is the converter pocket in the end of the crank. In the USA, the crank pocket changed size between 1967 and 1968. If you have the early small pocket, you will have to use the 18 spline converter. If you have the later large pocket you can use either converter. The 27 spline converter fits the crank "as is". The 18 spline converter requires a adapter bushing in the end of the crank.