Since it has the orig eng I'm more inclined to put it back original . I have a bad left leg & foot which makes shifting gears difficult
With such a rare/desirable car I wouldn't even consider modifying it. If you really want to build a pro car I'd sell it and look for a better canidate to build like a /6 or 318 Barracuda vert. Or buy one already modified. Modifying the one you have would certainly hurt it's value and would be a waste of a rare car IMO.
I understand your physical limitations may force a change, and changing it to an automatic but saving all the original gear would be a decent second choice, just do as little damage as possible while converting it and save EVERYTHING. Also keep in mind that a manual trans car is a much more desirable combo come selling time.