I would say make it what you really want. As far as drivetrain, I personally would go with the 400, but a 6 would be pretty cool in my opinion, and I bet it could draw a decent crowd at a car show. As far as custom interior or paint, do what you like, take a look at other cars on the internet and get an idea what you like and go towards that (I originally wanted to make mine completely original, but then I realized that white is a boring and plain color, so I spiced it up and swapped the top and body colors).

Another thing that I found to be really helpful is to bag and tag every nut and bolt you pull off the car. I cant tell you how many bolts ive lost on the earlier projects by just throwing them into a box or container. Also pick up the chassis manual, I found it the biggest helper the digging into my first '69. and dont be afraid to ask any questions, because there are tons of people here all experience levels willing to share thier knowledge.

Good luck, and lets see some pictures of the project!