Leak sealers

My 330 ford is leaking water out the back of one head i think about half a gallon on a 200 mile trip. I would like to weight till may to replace it. I thought about some sealer but it has a new rad and heater core. Has any one ever had a problem with stop leak? I have used bars before and it worked but i didnt care alot about the future of the vehical.

Leak sealers