I honestly don't see any problem with Indomitable Soul. Iron Will, as has been stated, is a flat 10% increase. Endurance is extremely campaign and DM dependent; I've had games where things like sleeping in armor were ignored, and campaigns where they were penalized. And Indomitable Soul provides a 25% increase. Widely varying benefits from Endurance (again, DM dependent), and a 25% increase on certain (and, yes, important) Will Saves, for two feats. I really don't see any major problem with that.
Figure its two extra feats for Fighters to pick up. That's another four levels of fighter for that character to make up those feats. If he instead picked up 4 levels of cleric, wizard, or any other class with a good Will save, he would have gotten +4 to his Will saves. He'd never get more than +2 to his Will saves from 4 levels of Fighter, so the biggest increase in Will Saves he's really gotten is 15% (and, if he was going to leave after just 4 levels of fighter, its only a 10% increase). All things considered... its a useful feat. I don't think its at all an overpowered or broken one.
Steadfast Determination, on the other hand, I don't know if I'd allow. My major problem with that one is that Constitution begins to govern two saves *and* hit points. 1s not being an automatic failure honestly doesn't bother me that much; there are some feats in Complete Adventurer that change which ability governs a save, which don't have the bit about 1s not being an automatic failure, but they also don't have Endurance as a prerequisite.