So, candidate for longest build ever?
Me being me, I decided to follow advice and run the 290xs on air for 5 mins to check everything worked and make the coolers easier to remove. Almost a year to the day later, off came the coolers!
Anyway, I've just finished sorting everything out. All went well in the end, here are a couple of pics:
Currently the CPU and both GPUs are running around 50C under load- is that good/bad/unexceptional?
So, barring everything blowing up at some unspecified future point, I would say I'm pretty pleased with how things have gone (several hours of fiddling/sweating/swearing aside!).
If anyone reading this doesn't know anything about PC building and wants to jump in at the deep end- I would say go for it! Just ask for help, read guides and don't press any metaphorical big red buttons until you're very sure what they do.
Thanks again for all the help guys!