The 53 and 54 are almost identical. The grills are different. I like them both. Personal opinion. The tail lights on the 53 are ugly. Again, personal opinion. But I think you can swap out 54 lights for the 53's, assuming you can find some 54's in good shape on a donor car.
The enclosed driveline had a tube rigidly attached to the front of the pinion housing on the rear end. The tube enclosed the driveshaft and had a ball and socket affair attached to the back of the tranny which housed the u-joint. The tube resisted the rear end "wrap up" on acceleration. This is similar to the third generation Camaro except the Camaro had a bar that ran along side the driveshaft to the back of the tranny. The torque tube works OK with the original engine and tranny. Assuming you are going to put in a SBC, you will want to change the tranny. So, you will have to put a different rear end under it. Probably a good idea anyway. Better brakes. More reliable. But, if you are going to change rears, you should change the rear springs. The original springs were designed to only hold up the weight of the car. The resistance to rear end rotation under acceleration was handled by the torque tube. The springs are prone to break anyway. Adding torque retention to them will cause you grief. There are several good kits on the market. Might as well put a MII under the front to take advantage of better front end geometry, disk brakes, rack and pinion.
