Ok, I got my Halo figs about 3 hours ago --- this custom/fix took about 90 minutes total. So I can see myself doing it with many, many more Spartans.
The McFarlane Spartan looked cool enough, but he had massive child-bearing hips. Looks like he's wearing Depends under his armor. The first thing I did was swap out his crappy waist with the one from my 25A MC custom (unfortunately, my MC had to die so that this one might live
). His torso & head were perfect for a 1:18 custom, IMO. I wasn't too thrilled with his gorilla arms and long-ass legs & big feet however - so I knew I'd have to fix those or replace them somehow. All his joints are boil-n-poppable, so it made everything extremely easy. First, my original 25A custom & my JvC custom:
Here's the first test shot with the upper body on my old MC's lower:
Then I went to work on the arms & legs to shorten them but still retain full range of articulation. On the legs I chopped off about 1/8" off the top of the thighs (cut at an angle), then I chopped off close to 1/4" off his ankles, then re-drilled the holes and Dremeled out some recesses in his ankles to allow the joints to sit in more tightly.
Then I did a quick size comparison:
Pop quiz: at 1:18 scale, how tall would a 7' character be? Answer: 4 2/3 inches. Guess how tall my new John 117 is?
For his arms, I didn't want to chop anything if I didn't have to -- so I popped everything out and drilled & Dremeled deeper recesses in the elbows & wrists. That took about 1/4" or so off the total length, which I'm happy with. They're still pretty long, but not so lanky and he'll be holding weapons all the time anyway, so it's not even noticeable.
So anyways - less than 90 minutes later (which includes the time it took for the paint to dry), I present to you my Definitive, Complete Spartan 117-- The Master Chief:
Still fully poseable:
In-scale with the Joes at last!
"Short people got, no reason. Short people got, no reason..."
And paired up with the awesome Pred custom that GIJoey made me:
So anyway, this thing was hella easy to make - all you need is a Dremel, a 25A Roadblock torso (and the upper part of his legs), a pot of hot water, and an X-acto Saw and he's all yours. I'm going to be customizing the other two Spartans above into a Magenta CQB (Lil' Chief's character) and a Brown/Green Mk IV (mine) - both 1:18 scale. The Master Chief was so damn easy there's no hesitation in chopping them up the same way.
One thing to note -- I had originally sculpted the "mesh armor" on my 25A MC's mid-section, so it transferred well to this new version. So I suppose there would be need for a small amount of sculpting to get it to look right, or you could just find a way to use the existing McFarlane lower torso.
Good luck to those of you who want to try it - it's a piece of cake.
Cheers!
Edit: -- added my quickie Cortana -- just a Microlady (forget which one) with some effeminate dude's head (since nearly all Anime-style dudes look like chicks
) 
I wish the clear spots were a light blue, but I didn't want to jack it up and lose the transparency. So I guess this can just be "dark Cortana" or something. I mean, she's got a ponytail now, so her program's changed a little bit, right? And at least she has elbows & knees, unlike the "light-up" version
simply amazing 
