With your gearing your 400 will do just fine with a 750 DP. A 750 vac. carb would probably get better mileage, but your RPMs will come up a lot faster with the DP. Which is more important to you?
An 800 DP would even work well too if you don't mind losing a little throttle response. It might make the most overall power.
You say that your engine is supposed to be a stroker motor. You can't stroke a SBC to get 400 cubes. Only 400 or aftermarket blocks will make it to 400 cubes. Read the casting numbers at the back of the block and then look them up online.