From what I have heard the 4 bolt version of 400 block isn't a strong as a 2 bolt one especially after a 2 bolt version has been converted to a 4 bolt configuration with splayed caps and bolts - but unless you're racing it it wouldn't make any difference I suspect.
Given the choice (especially for a truck) I would choose the 400 motor properly cammed over a similar but smaller 383 type - no replacement for displacement...
They both have loads of torque done up right but the 400 probably more - and as long as you make sure it is cooled right it should win the torque war and move that truck pretty well!
Randy