[Anthony]

390 ci mustangs and cougars used small in/out WR toploaders. I think Ford switched to the big spline when fairlane and galaxy drag racers were twisting input shafts. So a cobra, much lighter, running street tires I think would be fine for the most part with small spline Toploaders, especially if you buy the tranny already like that. If you are going to rebuild it, you can just change/increase the input shaft size cheaply without changing/increasing the output, as you then need to bore out the tail housing for the bigger shaft. If you are going to go all out, get the big in/out.

When looking at a used toploader, always inspect the insides. Obviously look for chipped teeth, but more importantly look at the small synchro teeth. Usually, they are prominent on first gear, and compare those teeth to the synchro teeth on second and third gears. If they are considerably worn or chipped, then those gears will need to be replaced even though the large teeth look fine.

Long ago at a swap meet, I was looking at a used big in/out toploader, along with another potential customer, peering into the case, and the owner told us the tranny ran fine, no shifting problems, etc. However, the synchro teeth on second and third gears were worn such that they were about 25% of the size of the teeth on 1st gear. I told the other customer these gears were worn and needed replaced, and the owner got real upset with me. LOL.

As far as gearing, like others here, if you go with a CR tranny, use 3.54 rears or something close. If you go with a WR tranny, go with 3.07 rears or something close. You won't be disappointed.