I've posted this on a ford truck forum, but you guys always answer faster 
1999 7.3L F350, 240k miles. I recently did the water pump and hoses due to seeping around the water pump gasket. While there, I decided to replace the overflow reservoir as well - it was old and discolored, but DIDNT LEAK! I'm now on my 3rd overflow reservoir - they are all leaking at the front seam. I don't think I have a blown headgasket or injector issues - no signs of white smoke ever, and there isn't any oil or fuel in the coolant (visual, I haven't tested with kits. Maybe I need to...). I've also tried a new cap, but it's starting to leak before I build any noticeable pressure - I can open the cap and don't get much pressure released. So - just bad luck on parts from Ford? All were OEM from different Ford dealers.
Pics
1999 F350 7.3 CCLB Dually. Stock except for DP Tuner and Gauges.
I dont have much help but I cleaned my old overflow with some gasoline and it looked 80-90% new.
Looks like a bad batch out there, try a different vendor.
Maybe? Supposedly the same tabs as the ones GM uses (or at least used to use) in their aluminum engines from the factory. I've used it in doses up to what's recommenced on the card in several vehicles with good results.
It's that, plastic welding, finding a good tank, or converting it to a non pressurized overflow. If it's not building pressure your cooling capacity is severely reduced (it will boil over at 212).
I think the easy button is to clean up your old reservoir and put it back on. If it leaks, there is some strange anomaly causing an over-pressure in the tank and blowing the seam. If not, you've found the problem, AND you have a clean reservoir.
After a certain period of time, OEMs farm out the production of their parts to the cheapest bidder since it becomes no longer profitable for the OEM to handle it themselves. My guess is that they farmed the production of that part out to a bidder that was a little too cheap, and your replacements are suffering from a manufacturing defect.
I strongly recommend against the clay tablets. Great for some things, not a diesel.
Diesels suffer from nucleate cavitation. The violent pressure changes in a diesel cylinder transfer their sound/motion through the cylinder walls. There are micro expansions and contractions in the walls that can cause adiabatic boiling on the water side of the cylinders. This can erode the iron. Any additive has the potential to worsen this effect... except of course the additive that is designed to combat it.
Worth checking the coolant pressure as it warms up, once the old reservoir is installed.
Wow, this is an old one haha. I ended up trying 4!!!!! New coolant tanks, all from Ford. 2 from a local dealer, and 2 more from different online dealers. They all leaked, and they all leaked in the same spot. By the third one I got smart and bench-tested them in my garage before installing them. By the fourth one, I got pissed and spent way too much money on an aluminum tank and I've been fine since then. 
Looks like I revived it.... but it was near the top of the list. Maybe the reviving party deleted their post?
Either way... glad it's fixed.
In reply to Curtis73 (Forum Supporter) :
I spotted a revival canoe just before your post and alerted the mods. They sunk the canoe, and here you are!
