clutch spline

[74 challenger]

is there a way I can find out the spline number without pulling the clutch ?18 or 23 does the gear box have a number on it?

 

[704406]

Extremely unlikely it's an 18 spline, as they were Hemi and 440 70-71 only.
You didn't say small or big block, but pull the clutch inspection cover, disconnect the clutch push rod to move the throw out assembley, and you should be able to measure the shaft diamater, and with some one to hand crank the motor, Trans in neutral, mark and count the splines.
There should be the car serial number on the right side pad on the Trans

 

[challenger6pak]

The casting number is the same for 18 and 23 spline cases. The raw case was machined for the different bearing sizes that went with the 18 or 23 spline trans. As stated you have to do a visual check.

 

[74 challenger]

sorry big block and thank you guys