The neoprene seal was introduced in 56. So 55 and earlier used a rope seal. You can use a rope seal in a 56 and up motor, but you cannot retrofit a neoprene seal to a 55 and earlier motor. The rear main cap is machined differently, deeper really, and the neoprene sinks into the cap on the earlier motors.
For as really permanent fix, have the block and rear main cap bored to fit a Chicago Rawhide #38647 full circle lip seal. That requires stripping the engine down to a bare block and setting it up in a Bridgeport mill, or using a fixture like I designed a couple of years ago that adapts a cylinder boring machine to the back of the engine block. Setup on the Bridgeport takes several hours, and is very expensive. With my fixture in place, I can be making cast iron chips in 30-45 minutes, and the whole process takes a bit over one hour to complete.