Ther is plenty of room in the front of the seat for a 40lb dog, but that may put a damper on available leg position options for you. With the D-rings that come stock there is not A suitable number to move the seat back...so you would need to add additional D-rings to do so. A dog of that size will fit behind the seat just comfortably, but there will not be an abundant amount of room i.e., roaming around, jumping from one side to the other...just enough to sit or lie down.
Weight capacity is not a problem. Not sure about whether the dog would be comfortable behind you. I have a friend who posted a video on YouTube entitled "How To Build A GoPro Deck Or Doggie Deck For An Inflatable Kayak". He built the deck for the front of his 300x. His dog is a bit smaller than yours, though.
Thank you for your interest in Sea Eagle Boats. While you would definitely fit in the 300x Explorer, it would depend on your leg length as to how comfortable you might be. The interior measurements are 8'9' x 16". If the measurement of your legs from your hips down exceed 48" inches, you might not be that comfortable in the 300x, and we would suggest the 380x Explorer, which has an inside measurement of 11'9" x 15" and can hold 2-3 Adults or 750 lbs. ASIN: B000SL0KB0. Hope this information helps you decide. It's a great kayak!
No, the length of the paddle is not adjustable.