The heads, manifold and water pump may be reused. I beleive the crank can also be swapped since they had the same stroke (3.75) and 3.00 main bearing size. You might run into issues with too high a compression ratio on the heads, but depends what heads you have. I have a pair of 1973 350 heads on my 400 engine.
You'll want to use the timing cover you have for that 68 water pump also.