[William]

First of all the L72 and ZL-1 were not RPOs. Any order for them was routed through Central Office for processing. Most Chevy dealers did not know [or care] of their existance.

The main reason there were no Camaro/Chevelle RPO 427 engine options was a GM Corporate Policy: no engine over 400 cubic inches allowed in cars less than full-size. I believe that was the reason Olds farmed out responsibility for the 455-powered '68-'69 Hurst Olds, which were installed by Olds.

There was a strongly worded Chevrolet Dealer Service Bulletin issued in '68 or '69 warning dealers of the possible business and legal consequences of modifying new cars including swapping engines not offered as an option.