Ok, so, finally I have squared this project away, after a lot of thinking and head scratching. finding info was the hardest part, so I'm gonna feed back into the community what I learnt and how I did it, so it may help someone in future.
first off, consider buying the brackets direct from a VW dealer, they are cheaper than the after market places. I used Marshall VW van centre in Bridgewater, they were really helpful.
The important thing I found for the positioning of the brackets was that they were 38 cm apart, and all parallel and in line. this is actually a lot easier than it sounds. The pressed indents on the floor make it really easy.
I used a 5mm drill bit and then opened the holes up to 12mm using a step drill bit, worked perfectly and easily.
The time consuming part is getting the access underneath, not difficult, just a faff and a bit time consuming.
for access I drove the van up onto concrete blocks(with little ramps) this gave me plenty of space underneath to drop all the covers and fuel tank(which doesn't really need to be fully dropped) also the heat shielding only needs access for the drivers side, I just bent it down so I didn't need to remove the braking system stuff. The rear exhaust box needs to come off, but thats no drama really, just a faff.
I have 4 full length rails, they came with 42 bolts in total, there are 44 holes, (11 in each, but 2 in the middle rails can't be used because you can't access from underneath)
I drilled the furthest rear holes, ensuring they were 38cm apart and all in line, then bolted the rails in, ensuring the front was all parallel and in line. marked all the positions for holes and used a large screw as a punch to mark the drill hole.
As for the brackets, I used 2 pairs of the single ones on the outer rails about half way along, the rivet holes are already there. The 2 different part numbers are for either side of the cross members, not either side of the van, which is what I thought.
The centre rails, I used 3 doubles for each rail. They must be for the kombi brackets, as there is no point for the caravelle rails, so I cut them in half and used them either side of the cross members. The only place I couldn't put one is where the spare wheel support bracket is. This is obvious once you are under there.
all other bolts were just fixed in, I did consider using large washers, but considering this whole system is fixed down with 42 heavy duty bolts, and 20 odd reinforcing brackets, any crash that would cause the floor to de skin away from the sub frame would be so devastating, that it wouldn't really matter! the front seats have 4 each! belt and braces in my opinion!
So here are some photos, hopefully they make sense,.
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