y'know I'm pretty tempted to just one up your heatran with a heatran, add a balloon and call it a day.
BUT THAT IS NOT HOW WE DO THINGS AROUND HERE (especially with how I never manage to get into OU)
Slowking @ Assault Vest
Ability: Regenerator
EVs: 252 HP / 252 SpA / 4 SpD
Modest Nature
-Scald
-Psyshock
-Fire Blast
-Dragon Tail
252+ SpecsTran Fire Blast vs. 252/4 AV Slowking: 22.8 - 27.1%
(FF activated: 34.2 - 40.6%)
252+ SpecsTran Earth Power vs. 252/4 AV Slowking: 25.3 - 29.9%
Regenerator ensures that you can switch in and heal off the damage any time. In return, Scald does a minimum of 70% to Tran. Boom.