Joseph Townsley had denied murdering father-of-three Frank Baxter as an act of revenge as part of a long-running family feud.
The jury found him guilty by a majority of a lesser charge of culpable homicide.
The 63-year-old builder admitted he was at the wheel of his seven-tonne Iveco truck when it left the road and smashed into the home of Mr Baxter's father Matthew Baxter, 44, last May.
However, giving evidence at the High Court in Livingston, West Lothian, he insisted the death of his sister's grandson was a "tragic accident".
Townsley, of Bogside, near Wishaw, was also found guilty of attempting to kill Mr Baxter's friend Alex Minto during the same incident, but the jury deleted part of the charge which alleged that he had struck both men with the truck.
He was also found unanimously guilty of attempting to defeat the ends of justice by fleeing the scene and abandoning the truck.
Judge Lady Wise told Townsley she was calling for a criminal justice social work report before sentence and adjourned the case the The High Court at Edinburgh on September 18.