@ chive
The question of playing Dragon Pulse or HP Ice is difficult to answer ... In my opinion HP Ice is the better option though you can hit Gliscor and Landorus with a Fire Blast as well the HP Ice is necessary for Garchomp, Dragonite and also the really unusual Noivern. Dragon Pulse would do more damage on Lati@s but especially Latias doesn't need to worry about that damage while Latios can be 2HKOt both with Dragon Pulse or HP Ice. Dragon Pulse is a guaranteed OHKO but the HP Ice also after Stealth Rock, so the advantage of HP Ice is (practiculary on the Scarf-Moveset) to surprise (non scarfed) Garchomp and Dragonite without full healthy. Landorus and Gliscor also take enough damage from Fire Blast but especially with Scarf > Specs it is good to have the HP Ice for killing them.
Also the question of playing Flash Cannon or not isn't easy, I would prefer a rockmove as Ancient Power on this Slot. Only for Sylveon, Gardevoir and Togekiss Flash Cannon isn't that profitable at all, practiculary because another often played Fairy called Azumarill only laughs about every Heatran thanks to Aqua Jet. (well you should never stay in azumarill, maybe for good predicted wow's while it belly drums, but that's a different topic ...) Ancient Power is the perfect Move for the often played Charizard-Y and Talonflame.
As a last resort I would play Fire Blast > Flamethrower or even Overheat because with the Scarf it takes the role of a revenger for the Team (in most cases), so you should not except a Flamethrower-Sweep through the hole Team ... Fire Blast has more power which is important as well as Overheat. The SAtk-Drop is viable for Heatran because in 99 of 100 cases it will kill the Pokémon and has to switch out or something that resists the fire-attack switches in wherefor Heatran also has to switch out because its trapped in the move.
What you discribe that it can OHKO many Pokémon so with good switch-ins it often gets a kill ... that works better with Choice Specs + Fire Blast while the better combination for a Revenger is Choice Scarf + Overheat imo.
I play my choiced Heatran like this, it outspeedes 72 Spd Gliscor, the rest should be very obvious:
Heatran @ Choice Specs
Trait: Flash Fire
EVs: 32 HP / 252 SAtk / 224 Spd
Modest Nature (+SAtk, -Atk)
- Fire Blast
- Earth Power
- Hidden Power [Ice]
- Ancient Power