I have a 1978 M5, is there a kit to install four point harnesses for the front seats?

Maule sells a bracket which mounts on the rear spar carry through tube to carry the retract unit. It is a bit 'clunky' but does the job though careful adjustment is needed to prevent contact with aileron cables.
The belt and retract unit can be bought from Maule or elsewhere. Three brands have been used over the years.
It is Very important that the lower hip belt be cinched tight before an imminent accident as the upper body accelerates foreward before the lower body on the seat. This pulls the shoulder belts which pull the 4 point buckle upward into the soft gut and away from the hip bone structure.
The bones are easy to fix but internal gut damage is not.

On a related note, what are the options for rear seat belts in the M5? I currently have the single lap belt but would like to change it out for two 3-point belts. I saw the inertia reel setup in an M7 but am wondering if there is a non-inertia reel type seat belt for the rear (figure lighter and cheaper)?

Hooker Harness might be an option

The early M5 has no centre mount for individual rear belts so it must be fabricated.
The right side rear mount is using the front bolt holding rear seat upper support tube. Again, one must fabricate for upper left belt mount.