I understand you now have a DHW coil in you boiler and you wish to use it. Good - thats less expensive hot water for the family.
But I have several rubs with your plan, First it will be very slow to recover and the aux heater will need to run allot - = no savings... Two you have no plan to limit how hot the tank of DHW may grow to and sending DHW out to taps above code limit of 125 is just dangerous. Others will say they do it because they know its hot - but what about a guest - what about a brain fart moment. Pink body parts are too precious to waste.
Now this might not be as cheep as I hoped but let me give you an "out of the box" suggestion... Your 40 feet from the electric heater. You must be close to a hot water tap at the other end of the house??? Place a tee near that tap and run to the inlet of the boiler DHW coil. Run from the boiler DHW coil out to the bottom (drain) of the electric hot water heater - leave everything in place. Now near the 40 gal electric heater - get a bronze circ pump form the big box store and pipe it to pump into the bottom port, Control that pump with a strap on aquastat on the OUTLET of the hot water heater.
Here is what you get. The boiler makes hot water that you store in your current tank and the aquastat limits the storage to 125-130 max out temp while production is in process. You will have a little loss while pumping your hot water loop - but you have to heat the house anyway right?? When you lift the tap at most of your sinks - since hot water is circulating - you have almost instant hot water. 40 gal of storage is usually enough and when the boiler is in operation above 140 recovery is good - when the boiler is operating above 160 recovery is great! Hi long demand can draw the 40 gallons out quickly because you are also circulating the tank and killing the stratification of the electric vessel but over all the other benefits may be nice to have.
here is how you wire your aqustat to open the circuit on temperature rise:
and here is how I did the pump
If your not near a tap - loop from the BOTTOM of the current domestic electric hot water heater to the (IN) port on the DHW loop leg of the boiler and out back near the HOT port out of the 40 gal heater. This will replace hot water faster but I recommend you or have a plumber install a mechanical thermal limiting valve on the hot water out of the electric heater to the house - because this method will send water heated directly from the boiler to the tap and that could be TOO HOT.. protect your self.