Hi,
we are almost in the same boat, not sure I can help you but we'll see.
I also have a z400, and did my first install yesterday with Maverick and i did a clean one today with Yosemite. Both works with minor issues I'm busy with.
Note that this is my first hackintosh and i'm still new.
FIRST things first, it won't solve your problem but give you info on its source : you can use "-v" bootflag
I'm sure you'll get something more precise that someone around here might understand.
(However, when i used this bootflag with the unibeast usb, I got "still waiting for root device" and stay stuck here, while I had no problem without this bootflag !!!)
GRAPHICS, since you did manage to have graphics during the install, I'm not sure the issue comes from the graphics, but if you have added a kext for your graphics in post-install (most probably in multibeast).
In this case i would try to delete this kext and repair the authorisations, it is easy and quite quick.
I do not have the same video card than you, so my experience is probably un-relevant for you but in case. My graphic card is a quadro fx 580, I have to use the boot flag "GraphicsEnabler=Yes" otherwise the screen goes off before reaching the install.
DSDT you say that you've installed a dsdt.
where did you got it from? have you created it yourself ? how do you know if it is appropriate ? etc? I haven't found any that seemed right (i only found some for z400 on super old post)
Personally I didn't use any dsdt and simply use the "easybeast" config to which I just changed a couple of boot flags (graphics enabler=yes, enableIGP=no usekernelcache=no), it worked.
I wanted to create my own dsdt, to have the cleanest install possible. So I extracted the dsdt but the question that i'm facing is what patch are we suppose to use ? as far as I understood, without this answer, doing a dsdt is risky if not useless.
ONE RULE
The only thing I'm sure about, do the least you could and do things one by one, one change at once.
maybe if you could write in detail your installation process i could help you more, as my z400 hackintosh is running.
Cheers
ps : do you have 4 or 6 ram slot on your motherboard, there are two models on z400