Header install 69 Dart

ease of install, stock type manifolds are the ticket if you have them or can find them
headers can be tough, the cheaper ones usually require at least the removal of the drivers side torsion bar
the more expensive ones mentioned above are easier and IIRC are installed from above but may require lifting or tilting the engine
leave the glasspacks out if you are doing headers they rumble good enough, just a good set of mufflers to quiet the idle but give up the noise when you hop on it
with all that said, a lot depends on your combo and your wallet and your ultimate performance goals
to answer B: yes, and I would bet you need new shocks