P0300 code why can we fix it?

I`m sorry about the mistake on your engine,

well if the cylinders 3 and 7 are close on the firing order check the crankshaft position sensor. Could be "missing' at that point, I saw that all the commun "missfire causes" were checked already but I have the "feeling" that is an spark issue, also be sure that the sensor has the correct gap.
Here is a little trick that will literally show you if a DIS ignition module and its crankshaft sensor circuit are working: connect a halogen headlamp to the spade terminals that mate the DIS module to the coils. A headlamp is recommended here because it puts more of a load on the module than a test lamp. If the headlamp flashes when the engine is cranked, the DIS module and crankshaft position sensor circuit are functioning. Therefore, the problem is in the coils.

If the headlamp does not flash, or there is no voltage to the module or coil pack when the engine is cranked, the problem is most likely in the crankshaft sensor circuit. On most vehicles, a bad crank position sensor will usually set a fault code, so use a scan tool to check for a code. Or, check the crank sensor itself.

Check the cyl. 3 and 7 with the light ans see if there is a "miss".