i need help soon.. electrical problem

Ok so i put a new alternator in my car and everything started up fine and all my lights were nice&bright.. idle sounded good too.. but after about 3-5 mins on, the cars headlights strobed out and they went out.. my taillights also went out and my 2 gauges lights too.. but my dome light still works!.. i checked the fuses and all of them were still good.. so now i dont know what is wrong with the car... its a 67 barracuda with a 340 in it.,they didnt have an alternator for a 340 v8,,, so it was between a 276 v6 alternator or a 383 v8 alternator.. so i bought the 383.........., any feedback, ideas or suggestions welcomed!!!!!!!!!!!!!!! thanks!!!