1/4 mile ET guesses
1st and 2nd gear seemed to pull good all the way to 5500 but it really seems to die when it hits 3rd gear. That is why I tried to run up to 6000 in 2nd the one time but as others have mentioned may have started floating the valves.
Part of what you are feeling is the over all loss of torque as you go through the gears. Lets just say your 318 puts out 250 ft-lbs of torque as an average from 2500 to 5500. Don't forget you also get multiplication from the torque converter that goes away as the RPM climbs.
HP is something you calculate from the torque and RPM measurement. Torque is what you feel pressing you in your seat. "Torque is what opens the twist off cap on your beer, pal"
1st gear: 250 x 2.45 x 3.91 = 2394.875
2nd gear: 250 x 1.45 x 3.91 = 1417.375
3rd gear: 250 x 1.0 x 3.91 = 977.5
Aerodynamics really don't come into play until 60-70 MPH, just about the time you shift into 3rd gear.
I think you are doing really well for the modifications you have done to the car. This might make you feel better: I have a 67 Dart with a 360/904/2.76 gear with M1 dual plane intake, Holley 750 Vac sec carb, cheap Summit Racing long tube headers and duals all the way to the back bumper. On the old worn out tires the 60 ft was a joke, but the best I could get was a 15.81 @ 86 MPH. With a stock 2BBL cam the power fell off sharply after 4500.
Round 2 I added 3.91 gears and a 3500 stall converter ( I found the original converter/flexplate was for a 318 which caused vibration, new converter and B&M flexplate fixed the problem.) Rear tires were new, 27" 255s. Result 15.21 @ 85 MPH. The deep gears and torque converter really showed the lack of power over 4500 RPM. All of the gains were from the first 60 ft.
You can swap ignitions and check for fuel related issues, but I don't think you will find they are holding you back. I wouldn't buy anything for the 318 you don't plan to move over to the stroker.
Comments on the Summit Racing electronic conversion kit. My car did idle and start easier with it installed, but I found out the "orange" box that came in the kit was retarding timing 10 degrees on the top end 3500+ RPM. I didn't find this out until later when I was trying to recurve the distributor. I would see all my 35 degrees in by 3000 and then it would drop to 25 degrees as the RPMs climbed past 3500 RPM. I bought a stock box from Autozone and the retard magically went away.