This trick won't work as well on datsuns, which are a very light car and can sometimes spin both rear wheels on takeoff, but it's worth a try anyway.

Over the years the right rear wheel is 'lifted' by engine/driveline torque and the left rear is pushed down. We only really know about the right rear lifting as it tends to spin the right rear on a hard launch in a straight line. With years and years of this happening, the right rear springs will sag a bit more than the left rear (over an inch in some cases on other cars). So then it is 'double' as bad, not only is the wheel lifted on acceleration, but it's already starting from a place where it has less downforce.

SO what you can do is (absolute cheapest option) swap the left and right spring packs over. this isn't possible on all cars, and I haven't done it on a datsun 1200 personally, but I think it would work fine. Then the left rear is that inch or so 'higher' then you launch hard, and th right rear lifts, and they even out, so you can get a much harder launch, even with an open diff. Of course you still would get better results from a locker.

This wheel lift under power, well it might be an inch or so under launch in first gear, but obviously it will be far less as the engine has less 'leverage' in the higher gears. So for a circuit racer, once you leave the start line, you'll have slightly uneven cornering (it will corner a little differently left and right hand corners) from uneven corner weights. In this situation, what I'd personally do is to swap the springs side to side, but then use a thin lowering block (as long as possible to help prevent axle tramp) on just the right hand side (maybe half the difference between left and right rear ride height) and try it with a few mm change in thickness until you come up with the most consistent corner exits - so that left and right hand corner exits feel the same, or as close as possible to it.

That will get you about the best you can hope for with an open diff. Interestingly, as far as rear spring/ride heights go, I'd actually make a suggestion to do this even if you plan to fit an lsd or locked diff. Test it with the open diff, and play with rear ride height/preload until you get it as good as possible, and then fit the lsd or locked diff.

You'll also tend to find that if you have adjustable rear shocks, you can get a bit better corner exit (it will probably not 'feel' quicker, so you'd have to really check lap times or something) by softening the rebound damping rate on the rears. This would help prevent it lifting the inside rear during a hard corner (not that it is a huge issue with datsun 1200s, though it probably makes a difference on a ute) Of course you can only go so far with that adjustment before it affects mid corner and corner entry too much. Like everything in car modification, there isn't some 'magical' setting, it will always be a compromise of one area or many areas to come up with the setting that gets the best overall lap times (or performance or whatever).

Speaking of suspension settings, and open diffs. You may have heard of HQ racing. It's done here in Aus and I think in NZ too? It started well over 20 years ago or so, and it uses an early 70s 'family' sedan (the hq holden) with 202ci 6 cylinder (with very few modifications allowed, and a controlled spec camshaft and no head porting) and 3 speed manual. They must run an open diff too.

Well about 15 years ago (maybe more) one of the car magazines did an article where they took a bunch of older musclecars, and some modern (for the 90s) day ones that were owner modified, and also a hq race car. They tested them on the 1/4mile drag racing, and also on a race track. Not surprisingly, the HQ racer, due to large car, small underpowered engine, open diff, only 3 gears etc, came dead last in the drag racing times. But when they went to the track, it was the fastest one there.

Now people could definitely argue that the driver was experienced as a hq race driver, and his car was well setup, and perhaps that some of the owners of the 'classic' musclecars didn't want to risk damaging their rare and priceless car that they spent a fortune on. You could say that about some of them, but not all surely.

But the point is that a well setup car, even with an open diff can get around a circuit reasonably quickly. The driver (I can't remember their name) was clearly 'good' but maybe not Michael Schumacher or Sir Jack Brabham good. But what they did have was the advantage of 'knowing' their car well, and knowing how to make the most of it.

On a related note, I used to speak with a mini racer who did hillclimbs mostly at the stage I knew him. Between him and his wife, they were about the fastest mini hillclimb competitors of the time, sometimes even being the quickest under 1600cc outright (which isn't a bad achievement with a mini a-series powerplant). Anyway, whilst clearly their car(s) were well setup as far as suspension goes, after it was setup properly, they didn't tinker with it, aside from checking it/maintenance and replacing anything in the suspension that was worn. And the bloke said straight out that he used to see some blokes adjusting this and that then trying to get it better, but it ended up they were spending all their time hoping that some magic new adjustment would transform the car, and they weren't spending enough time properly getting used to the current setup/capabilities. And he was certain that that was one of the big advantages he had. he'd gotten it 'dialled in' and then spent the timing mastering the driving side of it.

Point being, given the potential risks of running a locked diff in the street, well if I couldn't get an lsd for an affordable choice, I'd stick with an open diff, and focus on two things - setting up the car as good as possible (and that too can be done cheaply relatively speaking) without 'going nuts' on different settings day in day out and then practising/testing the car and learning its limits (obviously do this somewhere safe and legal)