Personally, I would use the "Plastic Welding System" from ITW Performance Polymers in the black double tube that you have to mix on something other than cardboard... I repaired a plastic motorcycle fairing with this stuff and it really does the job... I would pull the reservoir and really clean the area free of coolant... mix this stuff and go crazy forcing it into the break, if possible... you can sand it down after it dries...
Bill
98 SHO (slow and stock)
98 Explorer
91 SHO MTX lowering springs, stupid stiff shocks, ridgid subframe connectors, strut tower braces, polymer SFB, shifter rods (no cables!), & K&N filter.
95 T-bird lowering springs, Cold Air Induction w/ 9" cone K&N, Underdrive pulleys, Fog ligts, 16" SC rims, Limited slip 3.08's, & Bullit proof 4R70W.
87 BMW 735i lux-o-boat