Way too close for comfort for me. I would not do it.
First, you are getting in off what I assume to be a red-eye. Deplaning can take longer because people were sleeping, they are groggy and slow to get their belongings. If you're towards the back, you will be waiting.
Second, not sure if you are checking bags, but you'll have to wait for those too.
Third, are you relying on Uber or renting a car? Have to add time for rental car check in.
Forth, morning traffic if you arrive on a weekday.
Fifth, checking in that early at Dolphin. Most likely your room won't be ready. You'll have to then check in with bell hop to your stow bags.
Sixth, are you relying on Disney busses to get you to MK? That's tagging on a huge unknown factor. I love Epcot resorts the most, but my #1 complaint is bus to MK.
They are unreliable. I swear, you will watch 5 busses come and go empty because they are headed to Disney Springs in the early morning, and all you want is MK. It is infuriating. You may get lucky, but you may not.
Way too many unknowns to safely say you'll make an 8:40am reservation. I'd see if you could change it to around 10am. Then you won't be stressing out, and can enjoy your first moments in WDW. Use TP reservation finder if you cannot secure a later time. They will notify you if one shows up.