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.