seed bomb is indeed fine. We could even start a trend :D
Here's the MS set I think you should run:
Heatran @ Air Balloon
Timid nature, 252 SpA / 4 SpD / 252 Spe
Magma Storm / Sunny Day / SolarBeam / Roar
HP Ice/SR/EP can be used in the last slot, depending on what the team has trouble with. You might think it's weird to have Sunny Day on a sand team, but it has a lot of usefulness in that it kills Politoed better than your current MS. Does. It also transforms Tran into a temporary sweeper- as it's MS. OHKO most pokemon in the sun. You can also just switch to Ttar whenever you want to bring Sand back up.
Remember, the team has a lot more problems with Toed than with Tales- so giving Sun a bit of leway to totally overthrow Rain is the best idea here.