340 Nodular or 318 steel crankshaft

the way I understand it is that a steel 318 crank would be from around 66-67. It could have a 1.55" pilot bushing hole at the back of the crank for the torque converter. Most all converters on the market are for the later larger hole 1.8".

Check this out! It could save you a headache down the road. If it has the smaller hole, have a machine shop put it on a lathe and make it the bigger size.

good luck with your build!