Okay your reference looks like part photograph and part artwork. Let me see if these help, you're horse is in a classing racing/running pose.
So let's go with one of the greatest racehorses there ever was - Secretariat.
Unfortunately, due to the photo quality it's difficult to see that grounded foreleg.
Here's another one of Scretariat.
And another great racehorse, Cigar.
You can at least see their back legs and how far they bend at the fetlock. (the joint right above the hoof).
On the front leg, to have the right leg stretch forward so far the left base leg needs to be more vertical.
Look at the second pic of Secretariat and where his front two legs are, right is far forward and left is not quite perpendicular to the ground. Compare that to Cigar, the last photo. His right leg is descending and he's just starting to put weight on it. The left leg has gone long past perpendicular and that's where you get the greatest amount of flex in the fetlock. If it flexed that far when 100% of his weight on it alone, he'd pop the seminoid bone. Which many racehorses have done and have to be put down because of it.
Now, lengthening the neck will be a little tricky but it is do-able. If you cut the base of the neck where it joins the shoulders, you can put a longer armature wire there and join it with the other in the neck already. When you build up the neck, shoulders, and chest, that will help support it. If you have trouble getting it to stay when you first attach it, put a little super glue on the join and sprinkle it with baking soda. It will dry the super glue instantly and you will have cement - it bonds rock hard in an instant - I'm not kidding! lol!
The back - if you make a cut right at the hips and remove some of the belly and shorten him there, you can re-attach and again have quite a bit of clay to help support the repair.
And just a mention of the mouth, the lips will stretch but the actual mouth doesn't when the bit is being pulled on. Horses have a gap between their teeth where the bit rests on the gums. Pulling on the reins puts pressure on the bit and that puts pressure on the gums and the horse slows down and stops because it wants to move away from pressure.
Because your reference looks like it's part real, part drawing, I wouldn't trust those measurements absolutely. Try to find a photo of a real horse in a similar position and use those for measurements instead. It's a popular pose with photographers so it shouldn't be too difficult.
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