Hi Losadel

Happy to see another South African on this site, but sad to hear about your problem.

I personally would do the following:
1. Check to see if your distributor is not in the wrong place. Meaning, one tooth out.
2. Talk to a Nissan mechanic about your comp on the comp gauge.
3. Can't you talk to the previous owner of the bakkie to find out if he fiddled with the cylinder head?

If your comp is too high, you can let Payon make a custom thicker head gasket.
A mechanic I normally talk to and use specialize in BMW's. Because the cylinder heads of these cars are so expensive he use thicker head gaskets when needed.
He showed me a BMW in his shop that he used a 3mm thicker than standard head gasket without blowing head gasket problems.
Or you can buy a second hand cylinder head from a scrapyard that is still in good condition.
First play with your distributor and timing to make sure before going through all the trouble.