The 300 I6 is pretty popular in dirt track racing due to it's huge torque curve and indestructible crank. It produces more torque below 1500rpm than a 351W, a basically flat 240ftlbs from 700-2000rpm! The biggest problem is that the cam and intake setup lose power drastically above about 2500rpm. Comp Cams makes a good option that runs up to nearly 5000rpm, but it does lose a lot of 1000-2000rpm torque. I'd like to get better efficiency at cruising highway speeds, where I'm usually running around 2500rpm. But with current gearing I couldn't make use of a 5000rpm redline. So at the moment I'm not planning to crack open the engine. I'll probably end up restoring a 1940s pickup truck before I do that...
The C6 is really tough, even in stock form. The downside is that it consumes a lot of power, and has no lockup converter. So even cruising at speed it's still sloshing around instead of engaging a clutch like the E40D. Unfortunately there's no way to add a lockup to the C6, but I could sacrifice some low end torque gain by putting in a "tighter" torque converter. This would probably raise my MPG by quite a bit, but reduce my low end towing capacity. I haven't looked into it too much yet, but it's an option!