Another suggestion or two - both relate. If you can't relocate the front leaf pivot/bolt higher, then would there be room to do a dodgy and relocate the diff about 1-2 inches forward, by shifting where it locates on the leaves? might not be possible at all, esp with large tyres.

In the US in the 60s they did a similar thing, well technically they relocated the whole leaf pack forward a few inches. Essentially gives more overhang and higher %age of load/weight n the rears. naturally one has to measure things up - even if the wheels fit ok, the tailshaft length might be to the extent it will bind when you shift the diff forward at all.

By the same concept, they also relocatd the front wheels forward a similar amount (doing it by itself means a little mre weight on the rears, but combined with rear relocation doubles the effect potentially).

Obviously it'd be way limited on the rear of a datto 1200 (they cut wheel tubs, relocated the guard openings so they almost looked 'stock' - as much as possible (but obvious to anyone looking for it)) but even 1 inch could help. On the front there's obviously dramas wth the front lower lip of the wheel arch hitting the wheel with larger than std front wheels. Hypothetically if you alrady cut the guards to clear the tyres, if you went back to stock front ones, then you could move the front forward via adjustable caster bars. By the time you get 1-2 inches forward movement you'd start to push the inner lower control arm bush into therange it'd tear (or risk damaging the bolt, given long enough time, if it was a urethane style bush with even less deflection/compliance) - so it couldn't be done long term for a streeter, but hypothetically if you only adjusted it like that for racing (simply counting how many turns of the caster bar nuts to go back to std etc) it'd help with a touch more rear weight distribution even when standing still). Ironically, it'd also temporarily have a few degrees (about 1 degree for every inch forward you can shift the stub axle centreline) increase in positive caster, which would make it more stable in a straight line - nice for drag racing.

Last little tid bit that I found helped a bunch of cars at launch (and it will even work with a locked diff, though its benefits are more pronounced on an open/std diff. - basically the leaves on the right hand rear will sag a little more over the years - from the axle always tryng to lift the right rear under acceleration. If they raise the right, they push down on the left, which should mean iwth a locked diff, the overall grip would be the same, but for some reason it seems slightly better in practice, evne iwth locked diff, if the tyre downforce is evened out.

Anyway, the right sags, so swapping left and right leaf packs (on any car that is possible on, or coils on others etc) works wonders. That slight sag is reversed, so there is (whilst sitting still) more downforce/load on the right rear. When you add the right rear lift under heavy acceleration, that slight bias ends up evening out at the most crucial point - off the line. So that it is more even stevens. I'd actually go for that in distinct preference to new leaves both sides, which means static downforce is equal, but right rear lift throws it out of whack under launch conditions (not as bad as a sagged rear would, but still not as good as a sagged leaf swapped to the left would achieve).

On those cars where springs aren't interchangeable left to right, often one can 'cheat' and simply run a lowering block on the left rear (make the block as long as practical, so it contacts more of the leaves and helps prevent axle tramp/windup more than most el-cheapo one size fits all lowering blocks ). If spare time (a bit chunk of it) permits, the way to optimise it is fairly simple. Fit a single spinner/open diff temporarily, find somewhere safe and practice launches, then add a small thickness lowering block (choose the initial added lowering block test thickness to level out the rear for the first test). do another bunch of launch tests, then go 5-10mm per time with thicker blocks.

Basically you keep going and it'll get to a point where even with an open diff, it no longer just lights up the right rear, it'll get to the point that both of them try to spin (or even just the left rear).; Find that point and go back just a touch (2-3mm) and re-fit the locked or LSD centre. Another option is to run (if you can find one to suit) an air shocker on the right rear, and inflating it to maybe as little as 5-10psi at a time, till it did the same job of preventing the right raar losing traction at launch.