how many horses?

That cam won't produce the numbers that the roller cam used in the crate engine does. (Remember the crate engine uses a roller cam). Roller cams generally produce way better than flat tappet cams. Those X heads were great in their day but out of the box aren't quite with the magnum heads either for performance. The crate engine uses the magnum heads. Also your performer intake with a little 600 carb is going to hold you back some. It'll make good torque but HP will suffer. A stock 340 iron intake would possibly make more power and you probably need some more carb. More like 650-700 to make 390 HP. Don't get me wrong. I'm not saying it will be a dog by any means. I think it would run great in a street car. Especially your light little Dart.