I'll add to this. The September, 1968 P&A 34 shows the following:
66-68 ALL (283, 302, 327, 350) ... SPACER (13/32 I.D. x 21.32 O.D. x 1/2 long) ... 3877600 (Note that per other P&A's, this spacer was also used on Corvettes, Trucks, Buicks, Pontiacs, etc...)
69 Chevy, Camaro (8 cyl.) (exc. P.G., T.H. 350),
69 Chevelle (8 cyl.) (exc. A.T.) ... SPACER ......... (No description of size) ........... 3955204 (Note that per other P&A's, this spacer was also used on Trucks, Full Size, Vega, etc...)
Ed