"You can use a '68-up large-counterbore crank/engine in front of a '67-down small-nose torque converter but you must install a ring to take up the space between the TC nose and the crank counterbore or you will eat trans front pump bearings and seals. You cannot put a '67-down small-counterbore crank/engine in front of a '68-up automatic trans unless you have a special torque converter custom-built, or you build up a "FrankenTrans".
