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