23 spline 1.248 " ...plenty big

18 spline 1.417 " ...it's a beast

Comparison:

Ford Toploader 1 1/16 input (1.0625)

Muncie M22 1 1/8 (1.125)

just put the tko 600 in forget bout that old rattlin 833 truck trans

Well, then I would prefer a 1 1/8" 23 spline...

I have a n 18 spline in a 70 Cuda (amen to that third gear thing) with a 440 6 pack. I also have a 64 Fury with a 23 spline behind a mild 440. Beleive me they are two different animals and its not just shifter arm length. Not real sure what the internal differences are but the 23 spline is much more enjoyable and easier to drive. They do have different clutches Old style Borg and Beck in the Cuda and a Centerforce in the Fury. ( I like the Centerforce MUCH better!!)I would say your clutch and tire choice should be considered before deciding on a tranny. I think the 23 will work fine.The Cuda has a 3.54 Dana(originaly a 4 bbl car) The Fury tachs about 2800 @70 mph (3.23)TheCudaabout 3000 @70. If the math holds true that would put you car @ 3800 rpm @ 70 mph witha 4.10 gear. I think your real problem is rear gear selection.

Paul

Well truth be told, I was wanting to build my Charger similar to that of my father's 70 RR. It's an original 440-6pak, Dana 60 4:10 trak pak car. Thus, the want for an 18spline and a Dana 60 with 4:10 gears -- he's told me story after story about the fun he's had in that car and I thought it'd be cool to have similar B-Body's with the exception of the six pak setup -- might sound stupid but oh well

That chart is correct for the diameter of throwout bearing sleeve part of the front bearing retainer and is not the same as the input shaft spline diameter.