[Jack21]

Generally, longer spring with same spring rate gives softer ride. 300 - 350 lb/in is about right with MII based front ends with 351 based motor. Aluminum heads, aluminum rack, aluminum water pump, aluminum bellhousing, T-5 trans: lighter spring. Iron, go toward heavier. Remember, the lower control arm acts as a lever on the spring. The weight on the wheel is about 600 lb, but the load on the spring is about 900 lb.

The rear spring sounds unusually stiff at 450 lb/in. Solid axle should be in the 180 - 220 lb/in range because the weight on the wheel is nearly the same as the load in the spring. With an IRS, the load on the spring will be more due to the leverage effect of the lower contol arm.