2003 Chevrolet Monte Carlo engine replaced 3800 series II

I recently paid a mechanic $625 to put an engine in my car. His starting labor price was $400 but he said he would have to modify the engine to work in my car so he charged me an extra $225. The day after I got my car back the engine quit & I had to call a wrecker. The mechanic says he is not responsible & accused me of running engine hot. Thing is it never showed any sign of running hot. After the car shut off & I turned my switch off the temp gauge shot straight up & then the fan kicked on with the switch off. What would cause this? Im now out $625 and still no car. This does not include the price of the engine. This was labor only.