I was never a fan of additives to seal an
oil leak. Is it was a rope type seal it might help it. I doubt KC would have used rope on an aluminum block.
Rubber seals, I doubt. If the additive can make a rubber seal swell, then it is likely to harm the rubber. So it might help for a while and then make matters worse. Don't forget about valve stem seals and all the other places that rubber may fail.
BTW - FE have a plug in the back of the block, and the rear main cap seals to the
oil pan by sliding a nail into the seal to expand it. I recall many threads where the owner had to pull the flywheel before they could see exactly where the
oil was coming from. Although FE have a bad reputation for the rear main seal leaking, that is mainly do to the cast iron blocks sloppy grove depth cut into them. Ford was using a rope seal and it didn't matter. Modern seals do matter. New blocks are machined better, at least I would expect that.