NHRA 2013 rule book - I underlined the pertinent parts.
Quote:
SUSPENSION, Rear Stock rear suspension may be changed as follows: Lateral movement inboard permitted when rear frame and/or suspension is moved in for tire clearance. Mounting points may be moved in on axle housing and frame as required. In addition to stock suspension, a pivot below rear axle housing is permitted. Axle housing may rotate in spring mount; side movement of housing limited to 1/4-inch. Spring mounts may slide on leaf-spring equipped cars no more than 1/2-inch forward or rearward. See illustration in General Regulations 3:4 for acceptable pivot mount for rear suspension. Leaves may be added or removed. Rear leaves may be aftermarket fiberglass type. Rear leaf springs need not be same length as original. Coil springs may be replaced with similar configuration units; rear trailing arms may be replaced or removed when using four-link or ladder-bar suspension. Removal of original springs permitted. Coilover shocks permitted in rear suspension. Shocks may be relocated; minimum down travel on shock is 1 inch. Rear trailing arms may be replaced with OEM-type aftermarket units, round tubing permitted, 1-inch-minimum diameter. Bushing must be in trailing arms that allow suspension movement but keep parts from being noticeably loose. See General Regulations 3:4.
