avidemux's dialog box for x264 settings is pretty bad. It's been years since x264 added presets from
veryslow, but avidemux still makes you manually choose all the cpu-time vs. quality options. It doesn't even have a checkbox for more recent (but essential) options like mb-tree, or psycho-visual optimizations (AQ and psy-rd).
I honestly can't recommend avidemux (2.5.4 in Ubuntu 14.04) as a frontend for x264. It's pretty essential to have something with a slider for slow to fast presets. I made a test encode, and the important settings default to on, which is good.
It doesn't matter too much what you use for youtube, as long as you use enough bitrate for it to look good. (CRF 18 is usually visually transparent, you could go CRF 16 to be on the safe side.)
There aren't any settings that will make youtube use your encode untouched for even one of its standard resolutions, instead of doing a transcode. The encoding guidelines, are all stuff that's on by default in avidemux. Except the GOP of half the framerate, which makes no sense anyway, and is a horrible idea. They're going to xcode the whole video anyway, so using a 1/2 second GOP size is just going to hurt compression. Same for only 2 b frames. I'm sure their decoder can handle the max consecutive B frames h.264 allows. (which is probably 16, since that's what x264 allows.) x264's default setting of 3 b frames is apparently good. I think with that one, they're just making sure that mis-informed people don't disable b-frames and worsen the rate-distortion (quality vs. filesize) tradeoff of their encode.