Datsun cranks whilst forged steel are still 34 years old, the centre line of the pin isn't centred to bore, you have no fillet radii left on the crank pins once turned down and they still don't clear the inside of the block and that just the L18 the L20 has main journals 5mm bigger standard than the L18 that is too big already.Then you need a flywheel which is no problem just more money.
For what your doing there are 3 other options, 1. Scat cast steel FF1600 crank, looks the same as a standard 711M or 2. the cranks I had made, 4340 forged steel fully counterweighted for $2200 and if you want to go big I have 84mm stroke as well, we would need to get you new pistons though and the cam would have to be notched.

or 3. just have the standard crank crack tested, serviced and balanced.