A rebuilt motor is a not a new motor. If it was a new motor it would add value
That is true, but in some cases like with these Vortec 350s where you know it's a given the headgaskets and manifold gaskets are going to go out, I'd much rather have a rebuilt Vortec that has been rebuilt using quality aftermarket parts that corrected the original design flaws, and that have been produced using improved designs. Especially if the only original part remaining is the block, with everything else having been swapped out.