I'm gonna try to stay on-topic, which is: "can the rear seal be replaced without pulling the engine." But I may digress into "how to replace the 440 rear engine seal properly." There are many youtube videos, and plenty of advice on the net for this latter topic, so I will try to stay focused.
Yes, it can be replaced without removing the engine (entirely), but it will take 4 or more times longer, and you won't be able to test the seal like you could with the engine out of the car.
Back in the 1980s, I got so good at swapping engines in e-bodies, I could do it in a day. In fact, I think my record was 4-hours to remove and install a 440 engine in a 1970 Barracuda and taking it for a test ride. This was without a lift, but using an engine hoist, floor jack and jack stands.
There are many, many threads of discussion talking about the rear main seal on the 440 on this site and others. It is possibly among the most-common of topics, because sadly, it's a common problem.
In my opinion, if you want to fix this once, and only one last time, pull the engine.
Dropping the crank in the car is pretty challenging, and will require an engine hoist, and partially disconnecting the engine, any way you slice it.
If you pull the engine completely out, it may take you a full day (your first time) but the install will be a fraction of that time, because you'll already know your way around. Often, the biggest time consumers are loosening frozen bolts that had never been loosened since the car rolled off the assembly line. If your engine is a recent install, you won't have to deal with frozen/broken/stripped fasteners.
Once the seal is replaced, you can test the seal by tilting the engine (on the hoist) so the rear of the engine points downward. Adding enough oil so the seal is completely submerged, and leaving it sit like this for a day (or even two) should show a leak, if it's going to leak, while the engine is still out. If it leaks, you can resolve without the need to pull the engine anew.
