I'm not a pro or anything, but here's some stuff.
What ignition do you have? You may be hitting rev limit. The MSD 6AL has a rev limit chip if you have one of those. The little white deal with a number on it plugs into the side of the red rectangular shaped box.
If your car is dropping off in the higher gears and not in the lower you're probably running lean. Check your density altitude and let us know about were you are. Use this link.
http://wahiduddin.net/calc/calc_hp_dp.htm
and
http://weather.com with your zip code
Colder air, higher inHg, and dry air will require more fuel. More load takes more fuel also, thus the drop off in top gears. It is winter and I bet you're running lean.
Also If I run 110 leaded fuel in my 10.5 to 1 set up it will extinguish spark and blow unburnt fuel out the side pipes. I'm not sure of the specifics on my fuel though. It starts about 5500 rpm with the MSD 6AL ignition while having a .05 spark plug gap. 100 unleaded pulls like a mother up to 7000 rpm.