Improving my 360 combo. Need advice/input

Dave -
That poor motor needs to BREATHE. I believe the poor thing is being strangled by the intake and 273 exhaust manifolds. There's been quite a few posts on here regarding valve sizing and the consensus (as far as I interpreted it anyway) has been the 1.88 valves are good in order to keep up flow velocity and aid cylinder filling.
The 750 Speed Demon should be good since Demon carb flow is measured wet vs. the dry flow measurement method used by Holley. Where you want the improvement will determine what type of manifold you should replace the existing one with. The M1 is a good manifold but is built for higher rpm flow. Street performance cruising will be best served by a good dual-plane manifold.
Cam numbers sound like the PO wanted this thing to really run on the big end but he did almost everything he could to restrict it's ability to breathe when it really needed to.
By all means you need headers. The PO was trying to get that 360 to exhale through stock manifolds that were originally meant for an engine 75% the displacement of the 360. It couldn't exhale and that also restricted the ability to inhale since you probably couldn't get enough of the burned gases out of the cylinders.
That poor engine was simply confused due to the parts mismatch.