Yesm the 3.9 and 4.0 are nomially the same engine but the 4.0 is cross bolted and has bigger crank journals, the crank driven oil pump and runs on thinner oil. No reason at all why you shouldn't stick a 3.9 in and with a little effort the front end of the 4.0 fits the 3.9 anyway giving you the better oil pump.