When I had a 258 it would go 90 with the stock carb (33s, 4.56s & T-176, typically between 1,000 and 11,000 feet, I live around 6,000ft)
After injecting it (junk yard multipoint) it would cruise at 90 part throttle (33s, 4.56s & t-176, typically between 1,000 and 11,000 feet). (Best 6 that I have ever had, 4.0s and Buick v-6s don't come close). I got better performance and usage from that 6 than many of the v-8s that I have owned.
...a well prepped, well driven, vehicle should do well in any terrain, including the highway.
Carburetors became obsolete during the last century... do what ever it takes to get fuel injection...It makes bigger grins off road.