If you prepared a 2nd level cleric spell as an arcane spell and spontaneously converted it into cure moderate wounds, would it still be arcane? Spontaneous Casting doesn't mention arcane/divine in it's description (just "cleric spell"), so it would be most reasonable to go with it being the same type as the original spell, which was prepared as arcane. So you're casting a spontaneous 2nd level arcane spell...
It does lower CL by 1 for the spell, so you'd need an extra level and enter sacred fist at 7th or practiced spellcaster to enter it at 6th like you plan to.
So that's still two feats or a level...a pity you can't convert domain spells. IIRC Anyspell spells are still arcane, right?. ...would the divine spontaneity feat from complete divine work with spell domain's anyspell? Hmm...probably not, since anyspell is funky.