360 issues-need help!

i just got a '71 Dart with a 360 in it. the engine is supposed to be recently built. it has 10:1 pistons. weiand intake with a carter 750, crane 302h cam, mp electronic ignition with new accel super coil and super stock wires, hedman headers and some other stuff i'm sure i forgot. the trans doesnt seem to have much of a stall converter in it, if at all. the rear is the 8 1/4 with some sort of posi and 3:90 gears. the problem i have is when you nail the throttle from a dead stop or when in motion the car mis-fires badly. some throttle positions are okay and when it doesnt miss it runs like a bat out of hell. it also does not want to power brake, but if you torque it against the converter and then let off the brake and nail it it spins the tires hard and gets pretty sideways. then as the rpm's get up the car again will start to miss. i want to get rid of this problem , but i'm not sure where to start looking or what maybe the issue. any help would be greatly appreciated.
dave