without adding much more, there are things you can do to free up engine drag which will reduce your hp rating in essence, "gain" some back.
disconnecting any non-essential systems that either do not work, or you can live without to gain hp, A/C, PS, fan clutch, etc. You can go with an electric fan to get proper cooling. If this is for the track then you may be able to live without these items and gain something.
Of course, proper wires, distrib, plugs, airflow, timing, are are critical to engine performance, there may be some lurking in there right now.
The engine is an air pump, larger carb, more fuel, headers, cam, exh system, lightweight parts like flywheel, heads, and other system parts can make a huge diff on how the engine responds.
To get the rear wheel to hook up better, go with solid u bolts and leaf spring plates from a Mustang to minimize flex of the rear susp. The Cougar uses rubber pads inside the plates and it squishes back and forth slightly under load preventing rear end hook up as solid as it could be.
J