If it's low end torque your after, why not just put a 400 crank in a 400 block. As far as I'm concerned, in general and especially for a low end torque motor it would be most convenient and economical to start with as many cubes as possible. Good 400 blocks/engines are not that hard to find, and if you are not planning to spin a grunt motor past 5500 rpm or so, the complete stock internals(short rods and all) minus a good set of pistons would hold up just fine. Lots of low end fun! I run a 406 and love it. Big block grunt, small block weight!

Just a thought