Stock class rules 13.2.J

J. Driver restraints as outlined in Section 3.3.1 are allowed. Seats may
not be cut to allow for the installation of alternate seat belts or harnesses.
Passive restraint systems may not be removed. A horizontal
“harness bar” may be used as part of the installation hardware for
allowed driver restraints provided it has no more than two attachment
points to the chassis and is bolted at those locations. A ‘C’-type
harness bar may also be used. It may have four bolted attachment
points to the chassis (two primary, and two supporting connections to
resist rotation). Truss-type harness bars are not allowed.

Not exactly. Here is the rule:

3.3.1 Driver Restraints Seat lap belts are required in all cars, and must be installed in cars with passive restraint systems that do not include a lap belt. Installation and the use of shoulder belts or harnesses is strongly recommended, how- ever non-factory upper body restraints may only be used in open cars, cars with targa-tops in the open position, or cars with T-tops in the open position when two conditions are met:

A. The roll structure must meet either the requirements of Appendix C or the Club Racing General Competition Rules (GCR) 9.4.

B. The top of the roll structure may not be below the top of the driver’s helmet when the driver is in the normal driving position.