Hello AshokanKid,
I also hate unknown Mega whatever.
Have you considered moving to El Capitan directly, rather than moving to Yosemite?
There are two methods in common use to install El Capitan on classic MacPros, both use the "official" Apple Installer as their starting point. That being said, it means you need access to a "supported" Mac so that you can download a copy of "Install OS X El Capitan.app" from the Mac App Store...
OK, well I did as suggested, DLed the pre-patched Yosemite installer from Mega (this time to a Mac Pro, instead of my previous attempts to a Windows machine, which had issues) and once unzipped, did a "restore" of the dmg / installer to a spare drive, then used that drive to boot into my 2006 Mac Pro 1,1 (8 GBs RAM & original crappy 256 MB video card).
Then proceeded to install Yosemite onto a separate blank drive in the Mac Pro, which seemed to go (relatively) smoothly. In my opinion, it "runs OK", though a bit glitchy and was hanging on re-boot (hardly loads log-in screen).
I've been dreaming about and messing with this for a while, partly because there are a plethora of cheap(ish) 2006 / 2007 machines out there and partly because I just enjoy this kind of thing (re-cycle, re-purpose, etc.).
At this point, I am going to be happy with running Lion (or, even better, Snow Leopard!) on these older machines ("support" be damned) and here is why;
This is obviously a kluge / hack that does "work" and perhaps back when they first (sadly and abruptly) deprecated the 2006 / 2007 machines to leave them in the dust, many were quite rightly dismayed and looking for alternatives.
These days (2016) I see no compelling reason to run The Latest & Greatest OS simply for the sake of seeing it "run". For me, it is (and always has been) about pure and simple functionality and SPEED.
I would MUCH rather run Snow Leopard on a given piece of hardware if it allows me to run my APPS (my main reason for even having a computer, haha) like Logic Pro, Protools, Final Cut, etc, than to run The Latest & Greatest OS that gives me "features", eye candy, Visual Effects or embedded apps that I could not care less about and otherwise poor overall performance. Seriously.
Now, if one has a certain software that will only run on a full 64 bit OS after 7.5 (AND only has a 2006 / 2007 machine), then of course, this may be a good reason to go this route IF the machine runs the software smoothly and quickly.
If it is sketchy (my experience here, even the OS alone was a bit dodgy) then what is really the point? Now, to be honest, MY personal preference for an OS is something MINIMAL, like some of the Linux variants, Windows XP (still running great on most of my own home & office machines here), Snow Leopard / Mavericks or anything that is sleek and FAST.
I basically want / need (and always have wanted) a simple server type functional platform OS that runs my desired APPS solidly.
I'm not into putting lipstick on a pig, LOL, so to speak. Just being able to "run" El Cap, Sierra or whatever, gives me no satisfaction if most of my RAM & processors power go to simply running the OS... ;-)
/rant
AK