Whoa! 75-80 mph in a 20 series pickup...?!?!?! Assuming your rear end is stock you had to be turning around 4000 rpms!
If you don't have overdrive or a low 3-something rear end I don't recommend too many trips up in the high altitude rpms with an old 292. I won't even turn those kinds of revs with my newly rebuilt 292 that has light performance mods to it.
From what I've learned here running excessively high rpms for extended periods of times leads to connecting rod stretch, thereby shortening engine life. Not to mention the stress on a 73 pound crankshaft to keep up that many rpms.
Keep it around (or under) 3k and the engine will last you exponentially longer. What is your rear end ratio?
As an example my engine is happiest around 70 mph in overdrive. That's about 2300 rpms. At 80 it's still not complaining too badly, but turns just over 2600 rpms. This starts getting just out of the ideal cruise rpms recommended by the cam manufacturer and the engine begins getting fuel wasteful. And I've added a larger 4 barrel carb, exhaust headers, aftermarket cam and a more efficient HEI ignition.
-ftyler
