Part of it is that I'm not sure how far I can push the motor. It doesn't have a tachometer, just tics on the speedometer that tell you when to shift. So I have no gauge of how high it'll rev.
Any suggestions on how to figure this out?
The speedometer was mis-calibrated and is off by seven mph, and i'm not sure if the same goes for the tic marks, or if I should go by how they are now. I can go a bit past the marks without shifting, but I don't know if this will damage the motor.
First check if the throttle in the intake manifold reaches the full stop when you press the accelerator pedal fully. Use an (charming) assistant or put something heavy on the pedal. Do this with the engine off and don't forget to take the heavy object away before starting the engine.
If the throttle doesn't reach full stop, check if there is something under the pedal (like the carpet), otherwise you will need to adjust the pedal. There is a adjustment possibility on the firewall (difficult to see or loosen the bolt. Adjustment is done by loosening the bolt, pulling pedal up and tightening the bolt whilst the pedal is held in the up position, you'll need an assistant).
If the throttle does reach full stop, have a drive until the engine is nice and warm (15 miles or so), so that the engine oil has its full lubricating quality. Make sure that the oil is not old. Then test the full load speed. This is done in second gear and third gear, so when in second or third gear, fully press the accelerator pedal and see where the car stops accelerating. It must be around the second or third gear mark (two or three ticks) on the speedometer. (the marks are not shifting points, but maximum speeds for every gear, one tick for first gear, two ticks for second gear and so on) In your case around 7 miles above the mark, because the speedometer is not correct. The engine will make a lot of noise, it will rev at around 4300-4400 rpm at that point. If the engine cannot reach that speed, adjust the full stop on the throttle (it has an adjusting screw with a locking nut) until it does reach that speed. The engine can handle this, as long as it is warm (not only the coolant, but also the engine oil)
The marks on the speedometer only work if the tires are stock size and the differential is original. Is the speedometer mis-calibration always 7 mph? or does it increase with increasing speed?
Again: proper maintenance is vital, for performance: checking/adjusting valve clearance, fuel filters and oil change in the air filter and injection pump.