Here is what the Guidelines say in relation to your question. 61polara was referring to these guidelines for Chassis.
This morning, I had time to check online with my local discount tire store and they have several brands ("Specific brand is unimportant.") available for less. Just make sure they follow your factory documentation.
From Judging Guidelines, under Chassis, pages 2-23 and 2-24: (Highlighted text is mine in relation to your question.)
"11. Tires on all vehicles must be as specified by the manufacturer. Specific brand is unimportant. …Some manufacturers offered a radial tire - for specific vehicle(s) - as a factory authorized option to the standard issue ply tire. The use of red line radials on a vehicle should be accompanied with documentation. It is advisable to seek documentation from the vehicle owner when a tire size or type is in question. This is especially so in cases of limited production, specialty vehicles, and foreign made vehicles.
a. LETTER SIZE tires, i.e. G78-15, GR78-14, etc 1967.
b. RADIALS, i.e. 185R14, 175R13, etc. Domestic Cars. (Factory documentation is required for any vehicle with radial tires prior to the years listed below.) Some domestic manufacturers offered these tires as early as 1967, such as Buick and Oldsmobile. It is possible that others may have offered them as well. BEFORE taking any deductions, the Team Captain will ask the owner for documentation. (Note: European built vehicles can be much earlier) (Japanese vehicles 1967, possibly earlier).
c. LOW PROFILE METRIC RADIALS, i.e. P195/70R13, P195/70VR14 Domestic Cars as early as 1979. European cars 1969*, Japanese cars 1975* *Also refer to above Note when judging foreign made vehicles."