Currently in my garage is a 69 Cutlass show car that has been completely restored frame up (dads), and a 74 Corvette (mine). I bought the Vette last year, 100% original (aside from fresh paint) and unmolested, from the original owner. The engine has about 20k on its rebuild (80k on the car) and a compression check showed nearly brand new compression levels across all cylinders with very little variance.
Since Corvettes were gutted of power from the 70s-90s, it was time for me to get to work. With a seemingly very healthy bottom end, I threw on an Edelbrock performer 2101 intake manifold, long tube ceramic coated headers (also wrapped since C3 Vettes can get hot inside), a remanufactured quadrajet (original was ever so slightly warped), 1.6 ratio aluminum roller rockers, swapped to an HEI ignition with 40 ohm wires. All it needs now is some 58-64cc heads, and a real mild cam (don't feel like changing original torque converter). I'm fine with a low rpm stump puller. I dressed it up under the hood a little with plug wire looms, valve covers, and a shiny new air cleaner.
I've also performed a trans flush and coolant flush. Trans fluid actually looked pretty new when it all drained, but whatever. I refilled with Type F for tire chirps when shifting. The coolant on the other hand, WOW! It came out dark brown.. I saw the coolant channels in my block when I did the intake manifold, and the gunk I saw lead me to doing the coolant flush. Or should I say flushes, since it took two of them to clean it up.
On one of my first drives with the car, I broke a motor mount and the cooling fan destroyed the fan shroud extension. I'm running with no shroud extension now and the original radiator, and still have to fight the car or drive it an extended period of time to get close to 200°. I'm even running a plug two steps hotter then factory. In Corvette forums there are many people that have issues with their C3s getting too warm under the hood, and I don't really understand why. My question to them is, have you ever done a full cooling system flush on your 50 year old car? Because it made a world of difference on mine. I did pour a bottle of Royal Purple Ice in the radiator since I was warned of engine temps getting high on these cars, but I doubt a bottle of snake oil makes THAT much of a difference.