Wasn't the timing a bit off here, no pun intended? I think what you are asking, Neville, is whether or not a full-out effort on the part of Jaguar with the XJ13 would have been competitive in the same time frame? I don't know the chronology of the XJ13 as I do the GT40 and its derivatives, but it's not a bad question.
It's hard to answer, though, as there was only one XJ13 built and it wasn't extensively tested, to the best of my memory. I don't believe it ever actually raced, did it?
As far as competitive, well, I think it was John Wyer who said that in order to finish first, you must first finish. Ford's approach to endurance racing was logically sound- detuned engines running at output levels high enough to get out in front, and cars that could carry them around and around for 24 hours. And Ford was determined to win whatever it took. In order to beat them, Jaguar would have had to have had cars which were just as reliable (Mark IIs and Mark IVs were very reliable once sorted out) and fast enough to stay up there with them. I don't know how fast the XJ13 was, but I don't think it had the speed that the Mark IVs did.
As far as possibly winning in '68 and '69, I think the GT40s campaigned by Gulf were nearly as fast as the big-block cars by then. And they were reliable enough to win. Plus they had arguably the best race team manager alive at that time, John Wyer. Jaguar would have had to assemble all those elements and make them work.
Ford didn't win four LeMans 24 hour races because no other team was any better; they won them because they were the best at the time. Endurance racing isn't just a matter of speed; it's also a matter of driver endurance, organization, resources, and head games played against the opposition. I think at the top of their form Ford really were the best. They didn't just spend a lot of money- they learned fast and profited from their mistakes.
It's a pity Jaguar wasn't in there with a race team. It would have made an interesting era in motor racing even more interesting, no question, and if nothing else it would have made Ford work even harder for their victories.