I have just gone through some of this myself. I'll answer the questions I can.
PJD642 said:
- Edge & 9M bars are the same for all practical intents, at least as far a mounting 400 and 700 series LED heads, right?
I believe so. However, you can't get modular corner modules to mount in an Edge 9000 frame extrusion without first cutting/grinding off the lower inner mounting tab. It does not slide into the extrusion, as there is no channel for it:
PJD642 said:
- Ultra Edge bars are deeper front-to-back than regular Edge bars, and need different end caps? Any other differences?
Ultra frames are 2" larger in the front-to-back dimension. Edge 9000 frame extrusions lack the mounting channel for the modular corner modules as pictured above.
PJD642 said:
- What is the correct name for the additional mounting pieces I will need to hold the 400/700 series LED heads in place?
When I purchased some over the last few months, I used the word "standoff" in my wanted thread and had several people understand what I meant. There are some by that name for sale in the classifieds currently.
PJD642 said:
- Do the 400 series heads fit into the ends of the bar where alley lights normally are, or do I need special mounts for those too?
Yes, and there are no special mounts. In fact, the 9M uses 400-series halogens for alley lights. Ultra and 9M endcaps have clips and bosses designed to retain 400-series lightheads.