450 sounds like a lot for that car on a typical Mustang II style A arm. On the front, you usually want to use a lighter spring compressed some and use the shock to control it. On a deal like yours, I've tried tying them down to get a light and it made the car junk. You need some front movement to get some pitch rotation and an aggressive 4 link set up to make it go red.

Who's shocks are on the front? What is the installed C-C on the front shock? Is that 2650 with driver? Not all A arm setups are the same. Take two measurements - lower control arm pivot to lower shock mount and lower control arm pivot to center of lower ball joint.

What gear set is in the trans?