i have hered and i quote "If you want your 1200 to have some -ve camber (factory spec is +1.5 degrees), just move the lower control arm mounts in the cross member. Redrilling the mounts about 10mm outward from their original position will result in about 0.5 degrees of negative camber." is this true?

I guess that would work.
I slotted the holes on the strut tower about 10-15mm. I figure this gives me some degree of adjustment

it will work, but I don't think that 10mm will get you a 2 degree change. You should also plate it for strength.

should work, but it would work better if you were to slot the holes and then use eccentric bolts for adjustment. they are those funny looking bolts that have flat washers welded to then off center. that way when you tighten them up the large flat washer on them helps hold everything tight. IF you are still confused about it go to a front end shop and ask them to show you what they look like.
as an alternitive, why not just put in a longer lower control arm? as I recall the 79-82 B310 ones will work and are about 10mm longer.

i had the crossmember on my b310 sunny van redrilled,& it took it from 1 degree positive to 1/2 degree negative.the distance the hole was moved amounted to the same diameter as the original hole,so there was the same distance between the holes as the diameter of the holes.also,if you can,get adjustable castor arms,because putting more castor on the front allows you to run very little static negative camber,but will cause the outside wheel to 'fall over',as it will,giving more negative.this will lead to heavier steering,but that depends on how much castor you get.my sunny sedan has 1 3/4 degrees neg camber at the moment,but has 3 degrees neg castor,which causes it to go to about 3 degrees neg on half lock.some bmws & mercedes run about 6 degrees neg castor,but that is definitely power steering territory,even on a light car.

There are many ways to achieve negative camber.
I have rose jointed the lower arm by cutting and welding in a box section.

you can also elongate the tower mounting holes by about 14 mm.
this is clearly shown in the very good download on this site that was produced by nissan motorsport in 1975. lots of other great ideas also and proven in racing conditions. don't ever take shortcuts with saftey issues.
Go to the down load and see how the experts did it

Rose jointing is fine for a race car application however for road cars in most countries you will have problems trying to get a warrant of fitness or whatever the equivalent tests are. Cut, welded and even modified suspension arms can be illegal for road use. Redrilling the cross member is quite safe and probably the easiest way of going about getting -ve. There isn't really room to fit adjustable bolts without modifying the cross member because of the big flat head that they use.

rose jointing is a definite no-no in australia,were it is illegal to cut & weld suspension arms.though,funnily enough,a friend of mine once reverse sideswiped his 1200 at speed into a concrete wall,& because he had lock on at the time,the left front wheel hit the wall.this bent the crossmember,the chassis rail,the strut,the stub axle,the drag link & the castor arms,plus the steering arm & the ball joint suffered too.BUT,the welded,extended lower control arm was straight,& was used in the rebuild.another way to get neg on the front,particularly if going to bigger brakes,is to use 200b(810) steering arms.these are offset outwards(well at least they are in australia)& will give another 1 1/2 degrees of neg.you just need to change to datsun 180b(610) outer tie rod ends,which have the same thread as 1200 tie rods.

does anyone have a copy of the 'blueprints' that the 1200 competition suspension manual tells you to refer to?

I know a guy who has the blueprints. They are rather large so I'm not sure how they could be useful in this forum. I'll have a chat with tensandtwenties about what can be done with them. I think scanning them would be a waste of time as the whole point is they are 1:1 scale.
Sorry to be a downer on this issue.