some info on a 440

OK I usually post about a small block I have but I'm looking to purchase a 67 dodge d100 with a 426 that has been bored to a 440. I'm buying the truck for 1000 bucks says trans was rebuilt and runs really good but overheats and his buddys said it was cause it was bored to a 440. It took a little to start and when started I tried to drive and when shifted into drive it just rolled forward. I figured I need to warm the trans fluid a little since it sat for a while. My main question is Why does it overheat and what can I do to fix it.