Exactly enough points left for an extra Irongut

I'd take the Standard of Discipline for the Ironguts personally, which uses up less than a third of what you've allocated.
Personally I'd go with an extra hand weapon for the Firebelly but it's really personal preference. Hellheart is a great choice, though if you wanted to save some points for something else then I'd say go with a simple Dispell Scroll. At 1000 points your opponent is only likely to have 1 wizard, so the Hellheart might be overkill.
Edit:
Firebelly - lvl 2 - dispell scroll - extra hw - 182pts
Ogres x6 - full command - ironfists - 222pts
Ironguts x6 - full command - standard of discipline - 303pts
Leadbelchers x4 - full command - 192pts
Now you have 101 points to spend. That'll get you 2 units of 10x Gnoblar Trappers if you wanted some flank protection, or a unit of 20 if you want a bit of a tarpit... Alternatively you could add 2 Ironguts and have a pretty nasty 3x3 (with the Firebelly). There's a few options