23 to 18 spline posible?

This question is floating on my head for long time, and i havnt had it answered or have seen nobody asking it,

I got a 23 spline 833 out of a 383 68 b body car, and i really want to ask,

if is posible to conver this trany to a 18 spline unit whitout having to change anything but the input shaft and front bearing retainer?

I know some comes with diferent gear ratios,

but is there a company that makes the befie 18 spline to fit the 23 spline unit?

are the 23 spline internals and 18 spline internals the same exept for the gear ratios?

Thanks in advance!