It sounds as though you already have the heads off. If this is right, check the bores for anything unusual. I'm assuming this is the case if you are looking for the HP you say ... Not going to get that stock with a truck block. That means at least a cam, but probably a set of heads as well.
- If it truly only has as many miles as you say, you should still see cross hatch in the cylinder bores. Check all of the cylinders. No cross hatch, rebuild it.
- The second thing to check for is to ensure there's no ring ridge at the top of the bore. If you see a ring ridge at the top, you'll want to rebuild it, 'cause the engine has more miles on it than what you were thinking.
- The third thing to check for is if there is any scratches or scuffs in the cylinder bores. You'll want to check this from the top and bottom sides of the pistons. If you see scratches, there's a good chance there's a broken ring, which means a rebuild. If you see scuffs, that means abuse and will need rebuilt.
- If you see any pitting in the cylinders, it has seen some open valves and some atmosphere, and thus rust ... You guessed it, rebuild time.
If you have the pan off, you can Plastiguage the bearings. If they meet spec and look in good shape (which they should be with that little amount mileage), the bottom end should be good to go.
Realistically, though, if you have it all apart, busting the glaze in the cylinders, with new rings and bearings, is cheap insurance. Is it absolutely necessary? Absolutely not. If you want it to last for a good long while, you might want to consider dropping a just a little more money into it just to ensure its fresh. You can probably do it without any machining of the block (unless you want to really do it right by doing an align hone on the mains and square deck the block and ... You get the picture).
If you haven't pulled it apart, you can do a compression test on each of the cylinders to ensure they are in good shape. If you are seeing good pressures without too much of a difference, it's probably in good shape.