also if they used the old torque converter (i dearly hope not!), it would retain a large ammount of old fluid. definately flush out your trans cooler lines (the 2 small lines going to your radiator, on the driver's side.) you need to either flush, or replace your radiator. otherwise plug the trans holes in the rad. and get an aftermarket trans cooler. over-heating is the most common way transmission's die.
GET THAT OLD FLUID OUT!!!
ps- they did change the filter right?