For clarity, here's what the GCR says:


Main Hoop Bracing
a. Main hoops shall incorporate a diagonal brace. The brace
shall either be in the plane of the main hoop, or extend
from the top of one rear brace (described in 9.4.B.2.c) to
the bottom of the opposite rear brace. Automobiles with
mid mounted engines can have the lower mounting point
attach to the frame of the automobile within six inches of
the main hoop. In the case of braces in the plane of the
main hoop, the brace must span at least 50% of the width
of the main hoop, and at least 75% of the height of the
main hoop as shown in figure 12.