You say slant 67/904 - if the motor and transmission was from a 67 in a 65 GT, then the cable was definitely different and would have been a 66 & 67 cable which used a double nut on both ends. When a 66-later transmission is used in a 65-earlier car, the 66/67 cable must used so the speedometer end of the cable will screw on the speedometer.
If replacing just the inner cable, yes, cut to fit. If you need the outer cable including the transmission end, it will either be used or that new one you referenced. A 'gotcha' is also the speedometer pinion gear! They are unique to the 65-earlier cables and are generally not easily found. I snag every pinion gear I can find. You might be better off locating a 66-up 833 which then you could use the 66/67 cable. Hope I haven't confused you. If you need more info, PM me with your phone number and I'll give you a call.