Here is a copy-n-paste from the 2014 NHRA rulebook:
SUSPENSION, Rear
Must remain as produced, except for the following: Leaves may be
changed (added or subtracted) as long as overall length and
mounting points are used. Coils may be changed (clamped or
spaced) as long as stock mounting points are maintained. Solid
bushings in rear suspension permitted. Cars with rear coil springs
may relocate the upper control arm at rear-end attachment point.
Rear trailing arms may be replaced with NHRA-accepted OEM-type
aftermarket units. Aftermarket lower unit must be non-adjustable
and have bushed ends (no heims). Aftermarket upper unit may be
adjustable and use heim ends instead of bushings. Sway bar(s)
optional. OEM or aftermarket torque arm permitted only on vehicles
OEM-equipped with a torque arm. OEM torque-arm attachment
points at rear end must be retained. See General Regulations 3:4.
As Charlie stated above, if a racer showed up with lowers that have an adjustable spring height , a tech guy might take it all wrong and ...