Man that's a huge bummer. I have some other questions if you don't mind... did you happen to check the lifter faces for being convex (slightly curved out) before you installed them? What about rotation, did all the lifters spin freely and smoothly in their bores? And I'm not sure if it's possible to determine but can you tell if it was a cam lobe that failed first or the lifter?
Also for everyone else... I'm seeing a lot of posts about using solid flat-tappet lifters to eliminate this issue. What I don't understand is if the problem is in the interaction between the cam lobes and lifter faces, what difference does it make if the lifter is solid or has a hydraulic plunger inside? How is it that solid cams/lifters aren't failing even if they're still flat-tappet?
If someone makes a solid cam that would be appropriate for truck and towing usage I'd consider it for my 440 but I'd also have to get adjustable rockers... seems like every solid-lifter cam out there is designed for street/strip usage and has way too much duration and lift for my needs.
