I currently own 2 real D/Darts. I had another one (the one that's been on the auction block 10 times) that I installed a 2 barrel motor with the Mopar Performance P-4452761 cam and lifter package in it. It was a .450/.455 hydraulic lift cam. Also installed were an Edelbrock Torker intake, a Holley carb off of a 383 Mopar engine and a Mopar electronic ignition kit. Not sure of the carb part number. It had 3.91 gears in it. I'd go with 3.55's if I was driving it on the street. I was told by the restorer that the car ran great. Both the Isky T-3 and Schneider 256F cams look like good choices. Use the 920 heads if they are on the engine. That's what came on the D/Darts.
As far as the rear end goes, pitch the 7 1/4 and get an 8 3/4. Back in Aug of 1965 NHRA issued a bulletin allowing the use of the B-Body 8 3/4 with 4.86 gears in the 273 CI, 235 HP A-Bodies. Racers must have been busting them regularly.
My one D/Dart ran a best of 12.92 at Detroit Dragway and 105 at Detroit, Milan and National Trail Raceway. This was in 1966 and in D/Stock with 7" slicks and 5.30 gears. I have 4.10's in my other D/Dart (380 HP - 360 Magnum Crate engine) and hate it on the street. It's got pep but I'm always looking for another gear to grab.
Good luck on your build.