The Find a Grave memorial includes a photo of the tombstone and THAT has the June 18, 1760 date which is probably more reliable than something written in the text section of the Find a Grave page. I realize this is still not a primary source for birth, but the fact that the only sources that have been found all agree seems to make a strong enough case.
On the other hand, is there any source cited for the date of birth on the other profile? If not, then I would say to use the one that is sourced and also include a note about the other date in the bio. If you want to bend over backwards to accommodate the other manager, you could also check uncertain for birth date.
I agree with Chase - this is not sufficient to delay the merge.