Chevrolet's AMA specifications submission dated 2/14/69 shows 2-1/4" pipes the full length of the exhaust system, including the tailpipes, for the L-48 and 396's (vs. 2" for the L-6, 307, and 350/250), and for the Z/28 (chambered system) it shows 2-1/4" forward pipes and 2" rear pipes and tailpipes. I don't have the pipe specs for the Z/28 without the chambered system, but it should be the same as the L-48/396 system.