Well It sound like the dealer is doing all he can to help. It is out of warrenty and sometimes there are just errors that are not caught by assy line inspections. In a perfect world they would fix it free. But they are offering to 50-50 which means they will fix it for you at their cost. So it it will cost them nothing. If there was a run of bad blocks or something you could hold out for more..but. I have been around them for years and have only see a few casting errors in blocks/heads/transmission cases, as in any mass production it just happens
I guess it comes down too they said they would fix it for 3 yrs/36k and it did last that long. It didn't last as long as it SHOULD have, but its you truck and its just the chance you take when you buy any vehicle.
It sounds like you are doing the right things, firm in what you want without being a jerk about it. If you/familly have used the dealer often ask to talk to the regional guy with the dealer and be nice but firm. In the end take what they offer.
Dave