That's what I did with mine.
I too have a 1968 23 spline 4 speed meant to go behind a 383 engine.
I'm using it behind my 360 engine.
The retainer that came with the 4 speed is too big for my bellhousing.
I was going to open up the opening in the bellhousing, but hesitated.
I went online and found the correct one thru Brewers.