Ding Ding Ding!!! We have a winner. The crank hub diameter got bigger in '68. The '64-'67s had the small snout and '68 up had the big snout. Throw in the fact that the input spline count on the auto trans changed in '68 also, and you have a can of worms. There are 2 ways to make things work. Keep your small snout crank and find a '66-'67 converter and trans, or find a '68 up 273, 318, or 340 crank and use the trans/converter you have. You could also have a converter made with the '67-back snout and the '68 up input spline count. Me, I'd be looking for a 318 crank. There's lots of them out there, they are cheap, and until '73 they were forged. Cast or forged the '18 crank will get the job done and will be internally balanced. Hope this helps.