That's because they got ULP and smog sh!t in 1972. Drive a 1970 Z/28 with 360hp or even a 300+hp HG GTS350 and you'll soon know what a 350 can do. The 300hp HG GTS350 is no more special than a HJ 308, even has cast iron intake, makes the 308 feel like a starfire 4!
As a comparasin for under-rated, drive a 253. Then hop in a 265 Hemi and you'll be looking at the 253 as junk!

Having said all that, a 253 is a nice engine. As WB says a 4BBL, decent mild cam, new timing set and oil pump,cold air intake, Edelbrock Performer or HT-HJ manifold, extractors and HEI ignition with 3.55 gears it'll feel like a rocket. But add all that together at rebuild time, and buy a 308 block and crank and rebuild it using all the same bits and you'll never look back other than the extra fuel. Or use the money for a VN-VP EFI conversion with a 200,000kM old engine and it'll give you as much power and 5-12MPG better economy.