Go here and download this for a start as the chassis section will basically tell you what you need to know.
https://docs.google.com/uc?id=0ByCvxnH ... ZLd1V0Q3c&export=download

Found here thanks to Dime Quarterly.
http://dimequarterly.blogspot.com.au/ ... o-modifyand-digitize.html

This is a digitised varsion of the old How to Modify datsun 510 (1600) Engine and Chassis but the chassis bit is applicable to a 1200.

A few pointers seam weld the chassis rails, Diagonally brace the upper cantrail (the upper rail the guards bolt on to) where it meets the A Pillar (the vertical section the front doors hinge off) and most importantly find a 1200 photo with a Roll cage fitted and work out where the Roll cage will eventually mount and then plate that area with at least 2 mm ~2.5 mm sheet metal across the floor pan and up across the inner side of the inner sill panel to give a strengthened area where the main legs of the Roll cage will eventually mount.
Same for the front legs where they will sit on the front floor and again on the rear parcel shelf where the rear legs of the cage will eventually sit and remember that at the rear parcel shelf area you really need to continue the cage down under the rear parcel shelf to pickup on the rear chassis rails.

If you do this plating before it is needed you can then bolt in a Roll cage onto an already reinforced section ready to go when needed, Otherwise you face the risk of upending the car and seeing the Roll cage legs punched straight through the floor where they mount and I have seen that happen as most cages only use a relatively small section for the mounting plate usually 5 mm thickness and about 100 mm x 70 mm size wise at best which is OK if it bolts to an already reinforced section, But not big enough for a major roll hence reinforce the shell where the cage will mount, The total area for the Main Hoop mount ends up being about a square foot in area to spread the load over a large enough area to work properly

You'd think that.. but theres a few wack of paint, and old sealer to remove so would be more than a day(the sealer almost looks like Tar)

If they are already doing rust repairs and have the welder out it should only cost you another day to have it all welded up.

Stitch welding is a good way to start stiffening certain areas, the entire engine bay is a good place to start, then connect the front frame.rail.to the rear frame rail, then start at the rear suspention mounts and start stiching away from them, over all the best results you will get is connecting the fram rails, best return for your $

dude all i can say is good luck. I've decided to go the other way cos i feel i wont use the car enough. track car --repairs, welding[which i can't do]drain tanks[fuel] when car is standing plus plus plus.buggered my doors by removing inner panels [weight]Lexan windows removed and put glass back. In general datsuns sit on the road so yeah it's going to be quite a job.I kept on changing before i even finished the car --still have not finished. throttle bodies [zx6 bike] and gotech mini x management etc made almost 100kw on base map with leak by exhaust and lsd which was not welded [blocks it rests in [alfa diff].Every time you strip you replace it --strip int replace with cage and fire ext --yeah so lots of respect for those building their own cars.

Not looking at putting a roll cage in until she's no longer on the road. As I'm having the rust n dings fixed, it is easier to have the seams done now, if I can afford it. That's why I'm asking, as the body work is already setting me back the better part of 3k.

Bung in a roll cage too ... That will help stiffen up the back

Firewall where it meets inner wheel arches and footwell stich up the chasis to inner wheel wells too rear same from what I look at. Try strengthen brace across rear seat area.
Anywhere to strengthen and reduce flex

So there isn't just specific areas that need to be done without going overboard??

Bugger sounds like an expensive way to go.