For the transmissions, you have the standard 3 speed, a heavy duty wide-ratio 3 speed, and an overdrive version of the standard-issue 3 speed (all are column shift), the 4 speed is an SM-420 with a granny first (shifter mounted in the floor). Automatics were the 2 speed Powerglide, and the 4 speed TH-400. The way to tell if the shifter was originally there is to look at the height of the hump in the floor.
A factory 3 speed truck will have a hump that is only about 6" high, and the factory 4 speed trucks will have a hump that is 8"-10" high, I'm not sure about the automatics.
The truck is 79" wide at the door handles, and 207" from bumper to bumper. The shipping weight is 3500 lbs, so drivable would be close to 4000 lbs.
Your engine options are the 250 and 292 Inline-6, the 283 V-8 and the 327 V-8.
The standard issue rear axle was a non-Posi 3.73. The options were a Posi 3.73 or 4.11 and a non-Posi 3.07 or 4.11.
A big help for me when I first got my '66 was ordering the Restoration Package from Chevrolet and you'll definitely need the 1963 service manual, and the 1966 service manual supplement.
