You deff want to put your outlet from the heater into a fitting BEFORE your raw water pick up. This will create all the flow you want without needing a pump of any sort. Hope that helps.

I was looking at a heater core out of a Chev Safari. the one that is for the rear heater. Its an all in one unit as well. But I didnt look close at the dimensions. That one posted for $169 would be a good way to go too.

You may also consider adding a hot water shower. If your running wet suits you just throw that on and fill the suit with hot water. WHen your done your run fill it with hot water again, then run hot water on your feet to warm them up before you get back in the boat. That got our season right up until it started snowing.