"the resistor i have that i was gonna put on is 2.5 ohms. is that gonna be enough?"
For an ideal 50/50, 6volt/12volt voltage divider, the ballast resistance is the same as the coils LV Primary resistance. Typical of ballast resistors is say 1.25 to near 2 ohms. 2.5 sounds a bit high, but can work depending on the coils LV primary resistance. If the ballast ohms is too high the spark may be a tad weak, if to low the coil might run just a bit hot
"stated right on the coil is "no external resistor required"."
If its a 6 volt coil used on a 6 volt tractor THATS TRUE. If its a 6 volt tractor used on a 12 volt tractor THATS NOT TRUE. If its a 12 volt coil used on a 12 volt tractor THATS TRUE.