NormanSufficool

Concerning the Spark plugs, the NGK B6ES was original but the BP6ES will work too. It just has an extended electrode for emissions on later model engines (L16 on)

I once worked for a German mechanic who had jet reamers. I had the same problem you had with my Nikki carb on my Datsun L320 Pickup. I slightly reamed the jet but it still stumbled. Years later I found out that the primary venturi that screws in had pitting under it and I had to clean it up and epoxy the pits and install a new O-ring under it and reinstall. Now, because I reamed the jet, it runs slightly rich and can't find replacement jets. So, check the main nozzle in the primary for corrosion under it before rejetting. It just unscrews out (2 screws) and inspect it and the O-ring under it. I was a parts man for years at Datsun and Nissan dealers and I believe the OEM carb for the 520 was a Hitachi, not a Nikki. Is your carb an after market carb?