I'm getting pretty outrageous torque numbers with the 408s I build..at very low RPM. We have been refining the design of the engines over time. Couple of 'secrets' included:
1. The 408 is 4.030 x 4.0 bore and stroke. Stroke length is important, obviously. And while increasing the bore size would help too, the blocks can't take much more than .040 overbore
2. Comp Dual-Energy (265 DEH) cam grinds really produce at low RPM. Typically end up with the cam advanced 4-6 degrees from straight-up.
3. Intake runner CSA is very important to keep velocity high at low RPMs. They are getting harder to find, but the oft-dengrated Edelbrock SP-2P intakes are the best choice I have found for building a low-moderate RPM 'torque monster'.
The 'downside' (I don't think it is a downside, but to each his own..) is that you end up with a motor that breaks the 300 ft-lb ceiling at less than 1500 RPM and peaks at almost 500 ft-lb below 4500. And then runs out of 'juice' very quickly after that.