I suspect that Victor-Reinz makes a TH350 gasket. They are a a rubber or neoprene coated steel o-ring and are reusable; found same as an OEM trans pan gasket on my '05 F150. I have another car with the same transmission and it was a leaker until I pulled that pan and replaced the Felpro with the Victor-Reinz. It's a little more money but well worth it, especially since reusable. Just pay attention to your torque wrench setting when installing, and do your best to hammer flat any flange area that looks misshapen.
