23 spline v 18 spline
18 = big block trans
23 = small block trans
and yes you switch 18 to 23 if they have the same bearing like said earlier
the rest of trans can be the same but make sure they are the same ratio
as some was o/d trans and dont have the same ratio and gears wasnt install the way third was at the fourth place so...
I did a few of them before even put a 833 on cheby so friend could bit the hell out of it with out breaking trans but he broke the dif instead
for a fair price rebuikd kit look at mancini racing
http://chucker54.stores.yahoo.net/mara4sprekit.html
it contain all the bearing and synchro ring you will need and more
but for gears
http://www.brewersperformance.com/
or
http://www.zbag.com/
hope this can help you