Hi Andrew,
This is an interesting use of "loom" - Nick Cave seems to have used the verb (eg. to loom over something / to loom into view) as a noun. So the image is of a wide and great landscape.
For the bells, there's a little misspelling. It should read "pealed", which is specifically how we describe a ringing bell (especially a loud and clear sound). The mission bells would be the bells of the church, set up by missionaries.
Interestingly, another member asked about "mission bells" a few years back:
http://www.italki.com/question/88062