23 to 18 spline what it takes and how good i'll be!

yeap, first how much it take to convert from 23 to 18 spline and how good this will be?

and also how strong is the 23 spline unit?
would it be enough for a really hot street 440 that will be seen track with slicks once in a while?

it is a 72 swiger minitubed dana60 and guestimate 550 hp.