[tubman]

First of all, there is no "stock" 12 volt coil for a Mercury flathead; I presume you mean a standard 12 volt coil. The existing coil should work just fine with your new distributor. The distributor is just a high speed switch that excites the coil, so it's function will be no worse than with a stock distributor, and probably a little better because of the superior design of the Mallory unit.

One thing that you should take into consideration is whether or not you need a ballast resistor. Ideally, you want about 4 amps going through the points; anymore and they will burn and less will cause a weaker spark. Most 12 volt coils come in one of two varieties 1.5 ohms or 3 ohms resistance. With a 12 volt system, you can calculate which coil you need using Ohm's law, R=V/I (Resistance =Volts/Current (amps). In your case, you need 3 ohms totals resistance through the coil to achieve the desired current through the points (3=12/4). With a 1.5 Ohm coil you will need a 1.5 Ohm resistor.

In your case, I would be looking for a vintage Mallory coil to run. They look right and have proven to be quite robust. They sometimes even come with a bracket especially made for late model flatheads. I particularly like their "Best" coil.