Ken, that's exactly what one can do! Basically, you jack up both rear wheels and crank in "some" preload (a few turn but definitely NOT all that's needed to push things back to zero rear camber) with the spreader bar clevis. Then you lower the car and drive it for a couple of days. This lets the myriad spot-welds that make up the rear monococque readjust to the 'new' loads without buckling the fenders or cracking paint. Recheck the rear camber as before and add more preload if required. My '72 was bad enough that it took 3 tries to get zero camber with all adjustment shims removed.
An alternative is to find someone with a professional frame jig and use heat and hydraulic pressure to bend the whole rear of the car back to OEM specifications. This is much quicker but takes enormous skill to not distort the fenders. Wilkinson has a factory frame jig and there may be others. Ask around.
It's been a decade since I measured but I think if you weld in about 1-1/4" or so to both ends of short spreader bars- even aluminum ones, they will jam into the square openings provided. The thin bolt-tabs provided then only locate the bar, not attempt to transfer big cornering loads.
If you remove the rear wheels and look up there, you'll see correctly positioned spreader bar ends touch the upper rear shock absorber ends; that's what we are trying for. In an open-wheel racer, the spreader bar ends would actually bolt to the upper rear shocks AND the upper subframe.
Finally, I was at Larry Stock's Pantera Parts Connection shop in Carson City NV yesterday and he has a used Hall steel spreader bar on the shelf.... Call him.