Back in the Day my 68 390 GT (with a very aggressive cam, not the one I ordered) with 3.25 rear and close ratio 4 spd did 70 in 1st, 90 in second, 110 in 3rd. Never did max out 4th, but 135 MPH in that car scared the crap out of me! It had a tired worn out suspension, and bias ply tires up front! It was stable till I let off the gas, then it fluttered! (I shifted it at 6,800)
That was my fastest (straight line) Mustang, and the only one I NEVER lost a race with. I ordered the 306 duration 427 solid cam for it, but I'm sure they gave me the 320 duration cam (same as they ran as NASCAR). The engine did not have enough vacuum to open the secondary's on a 735 Holley, so we put an 800 on it with Mechanical Secondary's. Had no power under 3,500 RPM, then it pinned you to the back of the seat.
Beat all the hot rods up at College with it in the 1/8 mile, in the mid 70s, they could not believe it. They all had gears, traction bars, built engines, etc. The suspension was loose enough that I stayed close to them on the launch, they when the engine hit 3,500 I started gaining on them, when I banged 2nd at 70 I jumped 3 car lengths on anyone I was racing, and the race was over!
It would just walk away from anything I was racing, that body style was very light. I ate up a 440 6 pack Super Bee with Headers, traction bars and MT tires like it was nothing! Raced a built 383 Road Runner and let off when I hit 4th, was about 20 lengths ahead of him.
It was faster than my 428 (68) , 427 (70) and current Mustang.
I left the 325 gears in it because I was going to school in Rochester, 320 miles away. The car made the trip several times, till I wrecked it when someone cut my off. It did not have a forgiving suspension, I had just banged second and was "loose", and he cut in front of me and I had to slam the brakes. Did a 180 across the road and cut a telephone pole in half.