Engine Power

Currently it has tthe 7 1/4 rear, but I was going to buy a 8 3/4???? from the guy I bought the car from. I was thinking a streetable 3:23 to 3:55 in the car. Something that I can cruise in but go to the track if I want. I already checked the trans-go shift kit out and was planning on that as well as the cooler and a mild stall. I used to have a trans-go in my chevy 402/th350 combo...worked well. As for weight, I don't know what it will weight when it is done. Is 3500 about right for these cars (73 Swinger)?

The guys I bought the car from is a big mopar guy and he has a set of the 340 heads that were mentioned earlier. Will those work better than if I had my 318 heads worked and port matched with my intake? 350 to 400 HP is now what I am looking for.