The 30K~40K miles comment came from buying running cars for parts for the race car operation. The durability of the production roadster engines is nowhere near the 'L' series.
Engine Timing Components 2000 - Datsun Parts
Datsun Competition Preparation Manual v1.1
Be mindful that the Prep manual was written in the late 60s. Back then they knew of the short comings of the chains, tensioners, rod bearings, rod bolts and a host of other areas that need improved for durability.