Calculate the support requirements for a load bearing wall -
There is unfortunately no way of calculating this unless given blueprints of the house.  Even if we did calculate it and even we overdid it, for something this big no inspector will pass it without an architect stamp.
I can mention two things before you start this...

given the current construction I don't think the beam will be a big deal.
you will almost surely will have to have new footings created.   You will be made to dump a bunch of concrete on both sides and this could potentially be a much bigger deal than the header.   (I get that the current footings worked for 300 years but they probably won't pass code for new work)

(also jealous because they stopped selling that fajita kit near me)