I've seen this question a few times. Just for grins I'll toss this out. The original 400m was used in 70 or 71 ltd cars, not trucks. The importance to to the discussion is that year only, those blocks had sbf bolt pattern for the bell housing. It might be impossible to find one of those rare blocks, but there out there. The only parts that are custom only would be forged pistons, beefy rods or forged cranks. With all the long rods on the market, custom pistons, ( need them made any way ) and some custom crank grinding?? All the rest, heads, cams, distributors are cleveland stuff.
Is it worth doing. Other than getting very lite side loads on the pistons. No. If you have room to use this block go with a 460. Much cheaper parts and still way cool. Even better be really unusual and use an LS3. Great milage, tons of power and lots of " WTF" when you pop the hood.