Here are the original birth/baptismal records available for the McNeil/Dunsmore family. There may be more children, but not recorded.
You state William was born May 1, 1758 and you have a record of this. If this is correct then the McNeil/Dunsmore line is not yours. This William was born the 1st of March, baptized on the 3rd.
The only other recorded William born 1750-1760 in Renfrewshire is the son of James McNeal/Jean Walker and he was born Nov 1758.
03/11/1745 MCNEIL AGNES JOHN MCNEIL/CHRISTIAN DINSMURE FR65 (FR65) F PAISLEY BURGH OR LOW /RENFREW 573/03 0010 0061
21/12/1749 MCNEIL ARCHIBALD JOHN MCNEIL/CHRISTIAN DINSMURE FR102 (FR102) M PAISLEY BURGH OR LOW /RENFREW 573/03 0010 0098
21/04/1748 MCNEIL JOHN JOHN MCNEIL/CHRISTIAN DINSMURE FR84 (FR84) M PAISLEY BURGH OR LOW /RENFREW 573/03 0010 0080
30/03/1754 MCNIEL HUGH JOHN MCNIEL/CHRISTIAN DUNMORE FR161 (FR161) M PAISLEY BURGH OR LOW /RENFREW 573/03 0010 0156
30/03/1754 MCNIEL JAMES JOHN MCNIEL/CHRISTIAN DUNMORE FR161 (FR161) M PAISLEY BURGH OR LOW /RENFREW 573/03 0010 0156
30/03/1754 MCNIEL JANET JOHN MCNIEL/CHRISTIAN DUNMORE FR161 (FR161) F PAISLEY BURGH OR LOW /RENFREW 573/03 0010 0156
03/03/1758 MCNIELL WILLIAM JOHN MCNIELL/CHRISTIAN DUNSMOOR FR215 (FR215) M PAISLEY BURGH OR LOW /RENFREW 573/03 0010 0207
Here is the McNeil/Dunsmore marriage.
21/04/1748 MCNEIL JOHN JOHN MCNEIL/CHRISTIAN DINSMURE FR84 (FR84) M PAISLEY BURGH OR LOW /RENFREW 573/03 0010 0080
You can obtain copies of the original records, if they exist you can find on Scotland's People
http://www.scotlandspeople.gov.uk Good luck
Don