Katie, the Mill Creek 15 seems like the most suitable yak available in kit form for your needs. Although it has a 27" beam, where the MC13 has 29", only the 15 meets your minimum 300lb load requirement. It should be very stable despite the 2" reduction, and a bit faster and easier to paddle. Obviously, you need the large cockpit for your pup, and it is one of the few kayaks models offering that. The only other way to take a mid-to-large size dog along is in a double or triple, and that presents all kinds of complications. So I think you are on track with the best model out there for your needs. I built the Mill Creek 13 two kayaks ago (since then, an Arctic Tern and an Osprey double), so if you have questions I might be able to answer, let me know. And of course, when its all built and launched, we'll want to see a shot of you and your dog in it!

Gini