Nice. Yes, get the 400. Chances are that it's a truck 400 with a rear sump. It's likely that the C6 has the right bellhousing bolt pattern.

I would take a conservative, low buck approach to rebuilding for torque. The stock cam will work if you get the right timing chain. If you can degree the cam, that would be a huge help. After that, the challenge is tuning the carb and distributor. I think you can well over 400 ft/lbs of torque and probably an easy 250hp just by tuning it.

If you are a little ambitious, you can install 460 intake valves (2.09") and port the pockets. As Bill said, aim for torque, not horsepower.