For extra security you can put a 2 piece shaft clamp in front of the pss stainless sealing ring to prevent it from sliding fwd.
I installed a PSS while in the water. The original packing hose was pretty degraded but had been wrapped multiple layers of tape and had a slow drip emanating from the tape as well as the packing itself. West marine had the pss in stock and there was no availability at local yards for at least a month. I had to unbolt and slide the engine fwd onto wood blocks and I also machined a new shaft coupler to use with a flexible coupling.
I wouldn’t recommend going this route but it’s possible.