I found it interesting that the '75-'77 E58 is listed with dual exhaust. I figured the drop in HP/TQ between '74 and '75 was due to having to use a single exhaust and a cat, but I found
this post that makes me think they did actually keep the dual exhaust through at least probably '76. Not sure how the '77 E58 worked since it still listed dual exhaust and the only place it appears to have been offered was the F-Body coupe but it can't do a dual exhaust. And the '76 F-Body doesn't list the E58 as an option.
I wonder how many '77 E58 F-Bodies were actually produced. The literature is confusing and is certainly from the beginning of the MY so probably went through changes. But the only listing of the E58 in '77 also has an asterixis that says federal only, while the engine availability for the F-Body says federal, high altitude and CA. My bet is the only car you could get the E58 in that year was a non-CA F-Body coupe, if at all.
BTW, the '75-'76 E58 had different carb tuning per the Vaanth documentation. Might help explain the drop in power compared to the '74 E58?