Have you contacted them directly? I'm positive it can take a wider wheel than that. Just found this:
http://www.cachassisworks.com/attach...701_ds_web.pdf
Camaro/Firebird 1969 - 11-11.6” tire section width (275 = 10.8”, *285 = 11.2”,*295 = 11.6”)
Typical sizes include: 275/45-16, 275/40-17, 275/35-18, 275/30-19, and 275/25-20. *285 & 295 series tire
can be used with a carefully selected wheel backspace that centers the tire in the allowable space.