why are you bothering with anti freeze? Many just drain and have been for years. Nothing wrong with it. And the best thing to do is, warm boat up to full temp and give a good end of year run, Pull boat out of water, and then just dunk the RWP intake hose in a bucket of anti freeze till it starts coming out exhaust. The motor has to be hot for the anti freeze to displace all the water in the block.

For your situation tho, forget the flush pro thing. Thats ONLY for a pressurized supply. Disconnect the RWP hose going to the tranny cooler.