Probably too late but the new plank could be split down the middle and used as the outside on each side of the table.
The new plank has a lot of character and suggests perhaps two bench tops. Then, go get yet another plank to use as a filler.
Factor in wood movement as was suggested before by Checkered Shirt. There is nothing sacred about table tops and they don't have to have tight sealed joints with Dominos. I doubt if the wood you are using has settled down and each plank will go its own way with the change of season. It might be best just to skip the Dominos, allow something like a 1/4 inch gap for each plank and concentrate on the cross battens, with oversize holes and stainless steel washers. Seal the battens before securing them down.
If you are intent on using Dominos, you might think about some breadboard ends keeping the gaps and giving the Dominos wide berth to allow for horizontal and lateral movement but breadboard ends at least will keep the planks from curling (perhaps). It would give the ends a nice look and suggest that you know what you are doing.
