Hey dude, I woulda probably commented earlier but I hadn't seen the thread. I'm going through almost the exact same thing as you so maybe we can bounce some ideas off each other.
I'm doing a BG for my 120 gallon. Mine is going to be removeable though so I'm looking at using magnets to hold it in place. Its going to be in 3 sections that will hopefully butt up nicely together so you cant see the seem in it.
I also just finished my UGJ's. I love them. I'd suggest doing them and you can probably do it quicker than I did by benefitting from some of my errors. Let me know if you have questions about the UGJs if you do decide to go with it. I've got some unique ideas that have my UGJs working very well and keeping my substrate quite clean despite a couple dead spots *** created only because of the layout of my ornaments.
As for your quickcrete question. I'm not sure which one is best either. I'll probably be buying my quickcrete in the next two weeks or so if things go well, but it could be longer as I'm still designing my BG.
I too am having some caves but I'm forming them first by siliconing "boxes" that jut out from the BG wall and then just carving the square features of the styrofoam to something more cave like rather than carving a whole cave out. I dont have pictures yet cause my progress isnt quite where I want it to be for my first pictures yet. I'm maybe 15% done the background project at this point. I've got my firsts section almost structurally complete. Meaning that it hides my powerhead on one side of my tank but still doesnt look quite rock like enough for my liking. I'll be doing all the structure of it all then use "Great Stuff" to add more features so it looks rock like along with just carving the styrofoam. My base depth of my background is only 3/4" and the cave/powerhead covering section sticks out a good 10 inches or so but I've managed to make it look relatively natural. I'll post pics asap in the thread I started all about it:
http://www.cichlid-forum.com/phpBB/view ... highlight=
If you do go with UGJs you should be planning this right now since you might want to integrate them into your background. My background is going to cover the powerheads that run my UGJs so my tank will look a lot more natural. I'm quite excited. So if you want to do the same I'd say do your UGJ's first before the BG. UGJ's dont take that long to make. My BG is going to take me at least 3 times as long I figure by the time Im done.
As for where to put your intakes and returns... Im struggling with this a little bit too but moreso just with the intakes and my water heaters. I have an AC 70 as well and it just will waterfall over the BG on the back. The two canister filters I have both have returns that will still overhang the tank and over the BG. One is just a flexible return like in fish stores and the other is a spray bar in the top side wall of my tank. Both will remain after the BG.
I'm thinking of having my intake tubes hidden behind my BG and eventually appearing, but camoflaged, at the depth they should be. I guess I'll just carve out an area where they'll have to exist. I dont want to hide them much more than that so that my water is still being circulated properly and debris can actually get to the intakes rather than finding its way through some small hole or something.
Anyways thats a long reply. Let me know what your thoughts are about it all.