All good info. Owner needs to do some proper tests though.
Backfiring under load can also be due to a weak spark. Something Siteunseen mentioned in an earlier post about checking the points and condenser..
A weak spark will allow a car to idle fine, but as soon as any real load is put on it car will lose power and can backfire. Will also affect plug coloring. Not to say he doesn't have a fueling issue or Vacuum leak issue, but don't over look the Ignition system either. A weak condenser can be an SOB to find. Solution. Brand new points and condenser if old ones are at all suspect.
Don't know if OP has checked spark quality, but he should . Get a cheap plug tester ( less than $10 ) and open gap to about .060". With engine running you should get a strong blue spark and hear a bit of a " Snap". If spark is weak, intermittent or yellowish then you have an issue.
Same with fuel. By an inexpensive fuel pressure gauge and actually measure the fuel pressure. Better yet, buy a good combination Vacuum and Fuel pressure gauge. Quality tools and equipment are always a good investment and will last you a lifetime. Assuming that things are OK "because there is lots of fuel in fuel filter " is not the way to test.
Same with alternator. Put a volt meter on it and measure what it's putting out. Consider switching to an IR alternator down the road. Original Nissan regulators were pretty good in their day. But that technology is 40 years old now and the VR's available now are mainly Made in China crappola.
OP needs to take a methodical method of diagnosis and eliminate causes one at a time with the proper instruments. Otherwise you chase your tail in circles. Wouldn't be surprising if he finds several small issue.