Greeny speak to Dixie's contact from the past who owns Maddat enterprises in SA.
He can do all the work or guide you as datsuns are his specialty and he is passionate.
 
I would consider a well made roll cage to strengthen your van using small gauge but
thicker quality steel pipe or expensive but ligther cro mo.
 
Personally I would go quality alloy pipe to make it much lighter than cro mo at slightly
cheaper cost as theres no need to worry about cams or racing where its not allowed
but who cares as it will be mainly a road car with maybe occassional race duties?
 
Either way the using thick wall 1.5 inch diameter pipe would be all thats needed.
Yet if you do mind the look of a roll cage? it can be just a roll over hoop of 3 inch diameter
behind your front seats to also use for securing seat belts and hidden roll hoop under the
dash with sections going to your front chassis and also under the floor just infront of your
pedals to act also as underbody protection and used as gearbox brace mountings for the
stumpy gearbox without spending on a complicated frame just to secure the box as there
is nothing under there to brace to in the first place as chassis rail are combined with the
sills for original design which makes this area quite a weak section in the first place.
 
Also at the back can be use to strengthen behind the rear lower sill as well all nicely hidden.
Tubing in alloy is much nicer, lighter and nicer than heavy steel square pipe.
The alloy tubed frame underneath can also be used to secure roll bars and even exhaust.
 
Also if going for the trouble of DET make sure you run aftermarket ecu to get rid of the
air flow meter as this makes the conversion ugly. There was one online guy running a Honda
non airflow meter ecu on an American Nissan silvia or 240sx as the Honda ecus are
programmable and are used by the hottest tuned honda turbo and NA.
The HOnda ecu is cheap and then the softwares (2 types available) to tune the factory chip 
is much easier and cheaper than aftermarket options as its a factory quality ECU not built
but hand but by robots unlike low quality microtech and home made megasquirt range.