I looked at the pics again. Yours does not have the cross bracing behind the rear seat. That will help a ton, as will the rear frame bracing. You can also add the F41 suspension bits -- the 1.25" front stabilizer bar and the rear stabilizer bar. No bushings or links needed for the rear bar -- it just bolts to the lower control arms (though there are inserts for the bars, to keep them from crushing when you tighten the bolts). It also doesn't have the G80 RPO code, which means it doesn't have posi-traction. You may be able to find a posi rear axle out of a wreckers from a similar year. I had a Grand Prix posi axle in my '84 Cutlass -- 2.29:1 was the ratio (it went with my 3-speed).
That 307 should have enough torque to easily peel a tire. I owned two of them, and both were much stronger than their 140 hp official rating would have you believe. They'd be outrun by most 4-cylinder sedans today, sure, but no sedan today has the class those G-bodies had (and still have).
