Many of your statements are pretty subjective. If you've never driven anything like this, you just haven't got anything to compare to.

First, you can't change transmission ratios, only the RPM at which the transmissions shifts. This is affected partly by the "kick down" linkage, as well as what goes on in the transmission valve, and it's possible that there's something plugged there, I'm no expert on auto's, but for now, assume that it's maybe a combo of you not being used to the car, and maybe the kickdown.

Next, RPM versus speed is affected by tire size (diameter) and the rear gear ratio. There may be a (greasy) tag on the rear, and maybe not. Otherwise, block the car, jack up one rear, and put the shifter in neutral. Watching the driveshaft, and counting, rotate ONE rear tire TWO turns while you count the drive shaft revolutions. If, for example, the shaft moves about 3 1/2 turns, you have a 3.55 rear. If it moves 3 and about 1/3 turn, you have a 3.23, and so on. If it moves just a little past 4 turns, you have a 4.11, and so on. You can actually calculate how fast you will go with what RPM by using this ratio and the tire size.

I would buy or borrow a tach, and see what your RPM actually IS at say, 60 mph

You say you don't have much "take off power" but yet you think it's pretty low geared, which are not consistant. A low geared rear would give you lots of launch power

You really can't do much until you find out what you are starting with.

Just to give you an example, my old '70RR with 3.54 rear went almost exactly 70 mph at 3000 RPM. This means that at 6K, it was doing 140mph or a little more due to "tire growth" at speed.