I can give you a number of reasons why it's better to go in the intake side.
#1 send the mixing valve back, you wont need it. look at it like this, if its on the intake side, the plate exchanger may only hold 1/2 gallon, thats probably an overstatement, so only 1/2 gallon would be as hot as your furnace water, and by the time it goes into your tank and mixes with the 130 degree water. You wont know the difference.
#2 lets say your stove has a problem, wood doesnt fall down right, air vents get plugged up, been a long time since a fill up. the water temp drops to 120 or so in your boiler and its time for you and your family to get ready for work.. Guess what, no hot water. If you would have used your hot water tank as a storage tank, you would still have 50 gallons of hot water ready for immediate use.
#3 if the water entering the tank is 130 degrees and your tank is set on lets say 120, it will never have to kick on. and the more hot water you use in the home is the less likely it would ever kick on. hot water tanks today are insualted very well and use very little heat.
#4 leave your hot water tank on, ive ran mine both ways just to see if it makes a difference and it does not. but in the occassional circumstance where your stove gets cooled down, it will eliminate the case of this happening: stove drops to 100-120, your wife takes a shower and pulls in 80 degree water, it's not to hard for the electric to make up the difference and it keeps "anyone" later from having to take a cooler than normal shower.
Sooo........ Just leave it on, it makes for more consistant heating of water if something messes up.
I hope this is enough reason to make you want to put it on the intake side as there are no real advantages to putting in on the exhaust side. On the exhaust side you will have to run that mixing valve and then you'll have all the problems we talked about above.