Excess oil will have little to do with short cycle.
You are most likely low on refrigerant. 80% of R12 volume is a good start, but you need to charge by trial and error, but using gauges to see your progress.
The 124's are some of the worst cars there are for conversion, so you will probably have to experiment a good bit, if you get too much 134 in, the high side will sometimes start to "run away."
The speed sensor will not cause short cycling, it will simply kill the compressor and it will not engage again until you kill the engine then restart.