18 or 23 spline? Iron or aluminum?

Alright guys, I am putting together a 1971 Duster. It will have a 500 stoker motor, I would like to install an 833 transmission. I have found two 4 speeds to choose from, which is better the iron or aluminum case? Also since I can't seem to find a 18 spline will a 23 spline hold up to my motor without any problems? I have included the link of the two available and am going to purchase one of them tommorow, please let me know which is better for my application and what to look for.

Author: admin