A "front clip" refers to the front section of the frame, including everything from the front bumper mounts to the second crossmember or trans mount, depending on frame. Usually when referenced in modification terms, it means the front frame section from a donor vehicle that is used to upgrade an older vehicle, such as installing a later independant front suspension (usually passenger car based) to a vehicle that originally had a I beam axle. Sometimes, it's done to "upgrade" to a more modern independant front suspension to a car that already has an independant front end. The procedure in either case involves cutting the front of the existing frame off, then welding on the donor clip on. It's actually more complicated that that, involving cross measuring and getting the angles correct, but that is basically what it is. I'm personally not a fan of it. As to your truck, being a '63, there is no need to swap a clip in, you already have a superior suspension than any kit can provide. Upgrades are not bad on the '63-'66, as components from the '73-'87 can be used to upgrade the brakes. If you search the site, doing the brake conversion using '73-'87 parts has been covered extensively.
To answer the question about the signifigance of the 12 bolts, in GM terms, the 12 bolt is the strongest diff available (car or truck) without getting in to a 3/4 ton or larger trucks. It is so named because of 2 things; 1- as you alrady have noticed, it has 12 bolts holding the rear cover on, 2- internally, it also has 12 bolts attaching the ring gear to the carrier. Nice thing about the 12 bolt is that parts are still readily available. There is a minor problem with the early ('63-mid'65) diffs, and that is the axle spline. The early diff used a 17 spline axle, same as its predecessor. The late ones use a 30 spline axle, which is much stronger. There isn't anyone making the 17 spline specific parts 9axle shafts ans side gears), but the 17 spine can be upgraded to a 30 spline by swapping out those parts.
The GM 12 bolt is considered to as strong as the semi-float Spicer/Dana 60 (same axle used under the Dodge/Plymouth Hemi cars).
