Weird, if I build that in the late 80s with standard safety and 4.0ltr iron block V8 I get around 1200kgs (about 2700lbs), which almost sounds a little too light to me. (about 100kg lighter than, lets say, a Fox Body Mustang)
If I build it in 2017 with advanced safety etc, it’s more like 1650kgs (3600lbs), but that seems pretty reasonable for a modern muscle car of that size. (A modern Mustang is about that weight)
What were the similar bodies you were using? It might be more a problem that the others are too light, than this one is too heavy I suspect.
Regarding the crashes, let us know as much info as you can about when it crashes. What errors if any are showing up, and the specs of your PC. That’ll help us track down issues like that.