New to the Forum - 1968 Dart Post

Hello - I am new to the forum and wanted to take a quick second to introduce myself. In the past, I have primarily been focused on B-Bodies, so I have a lot to learn when it comes to A-Bodies. I have some great friends that are A-Body nuts. I have learned a lot from them and look forward to learning a lot from all of you too.

My 15 year old son and I are getting ready to build a 1968 Dart post, so that he can participate in his High School Muscle Car Club and the High School Drags (Young Guns). We are both very excited, me because I am a long time muscle car enthusiast and him because he is a teenage boy who loves cars, trucks and going fast. The best part of it will be making some fun memories together.

We are starting with a blank canvas. The Dart is a 2 door, post with a slant six, auto. It is pretty rough, so it is going to require quite a bit of work. Our goal is to build a decent looking, safe, streetable bracket car on a budget. We are thinking 360/auto with a 3:91 sure grip 8 3/4 rear end. We should have the car by the end of this month and will then start hunting for parts (large bolt pattern front disk brakes, K-member, etc.).

I have a couple questions right out of the gates for all of you (keep in mind best bang for the buck).
What is the largest 15" rear tire that will fit in the stock wheel well?
71 LA 360 vs. Magnum 360 engines?
J-heads vs. Magnum or Magnum R/T heads?
727 vs. 904 transmissions?
3:91 or lower gear ratio?

Thanks