If it’s the original motor, rope. The neoprene seal was introduced for the 56 model year. The rub is you can’t use the neoprene seal in a 55 motor. The seal area of the rear main cap is cut deeper so you will need to use the rope if the motor is a true 55 block. If the motor has been replaced, the casting date will assist you. What I mean by that is that the 56 production started in late 55. So you can have a block cast in 55 that has a neoprene seal because that block was an early 56 production block (think like August. or September 55). So take a peek at your casting date.
Now, there are a couple of rubs with the rope seals. First and foremost, the original seals were made with an asbestos component. The new seals, all of them, use a fiberglass graphite material. The old style rope seals work better then the new seals so if you can find a decent and still pliable asbestos seal, you might consider using it.
Notwithstanding, neither new or old rope seal is going to be perfect. They never were and so they all seem to drip a bit. If you are old enough, you may recall the black stripe that ran down the middle of every concrete highway. Millions of cars, millions of drips. We have gotten pretty use to no dripping these days but when these trucks were in their heyday, they dripped.
