LS swap Circulating coolant heater hosing

[Zinkedoo]

I got a 1000w Kat circulating coolant heater for my 5.3 lm4 in a 94 chevy k1500. Wondering where to put the return line from the heater? I have it going to the heater core return but when I plugged the heater in, the water pump weep hole started to leak. Unsure if the leak was there before or not. Image below is where the BLOCK heater plug is, the location doesn't work since it gets in the way of the motor mount brakcet.