I put the BBK clutch quadrant / firewall adjuster / cable in my '94 GT. I'm not sure if the "ball" on the end is bigger, the cable is definitely larger than stock. The clutch fork end of the cable on the BBK is a threaded rod with an acorn nut and a jam nut. If the acorn nut wasn't big enough for your clutch fork, you could always try to find a larger acorn nut with the same size thread. If you can't find the larger acorn nut (with the same size thread), hopefully you have a friend that is decent with a lathe and could cut you one out of some steel hex stock.
I would advise against attempting to solder on the clutch cable to build up the diameter of the acorn nut (or sphere depending on cable). I don't think it would last many miles with the forces that a clutch cable sees. If you tried the solder method, I don't think it would be long before you would have to get good at clutchless shifting at the right rpm.