HDK coil over system - spring selection

for stock K-Member. Have it already and also the qa1 shock multiple adjustable.

My question is about selectiong the correct spring. I know I have too weight the car. The whole car or just the front?
From that Point i can follow the qa1 instructions and calculate the rate with weight and angle...

But - can I have a longer length than the 12" Shocks to give more travel in expanding for weight Transfer ( drag race some times ). regular spring just longer or high travel spring? Hope that doesn't effect streetability too much?

what about springs with multiple spring rates they offer? any good or not recomendable?

car is a 72dart with 6.1 HEMI

Coil-Over Springs | Street Performance, Drag Racing & Circle Track Racing Coil Springs | QA1 Suspension