Front suspension set up and wheel alignment
Unless you are really dedicated to drag racing, why settle for the rear of the car sitting out of level?
A car set up for handling can still be a good drag race car.
A car set up for drag racing is often a crappy street car.
Me? I'd get the rear to sit level, then go on with the front end work. Aim for alignment with at least 2 degrees of POSitive caster, At least 1/2 of a degree of NEGative camber and about 1/16" of total toe IN.
My '70 Charger has Mopar Performance HD leaf springs, 6 leafs per side and it sits even. It is NOT set up for drag racing but does launch well and goes straight.
I have offset upper control arm bushings which allowed me to get over 5 degrees of Positive caster! The car steers like a new car with the alignment and the Borgeson steering box.
You CAN make these old cars feel and handle like a newer car if you choose the right parts.