It dawned on me that if James was their 5th child, then Robert would be the 8th, so I took a look at his baptism record, and he was indeed their 8th child.
James Moffat in Templehall and Elizabeth Story his spouse had a Son born on the 1st and baptized on the 21st February 1768 named Robert, being their 8th child. [Cambusnethan Parish Register]
Both records say their xth Child so my speculation about a previous marriage was incorrect.
Therefore they had four children before James in 1763. Therefore they were not married as late as 1761. Assuming a 2-year interval between births, the older children would have been born in 1755, 1757, 1759 and 1761, so a reasonable guess at their marriage would be in about 1754. (If there were any sets of twins or a set of triplets or quadruplets that logic would not of course apply.)
As Elizabeth's youngest child, (indexed as Elesebeth Moffet), was born in 1773, Elizabeth Story cannot have been born before 1723 and probably not before 1728, and she was almost certainly not born later than 1740. I cannot see any obvious baptism for her.