[olddog]

I agree with the OP that this failed slowly over time. The picture of the edge shows beach marks where a crack was formed and the metal was flexing, for quite a while. Very little metal was left at the time of the final failure.

I would expect everything to do with a frame to be mild steal. Heat treatment makes metal harder, and brittle. Hard makes it stiffer and flex less, but not necessarily stronger. It's brittle like glass. Mild steal is flexible, weldable, and bendable. All the things you want in a frame.

I have no way to say for certain, but my guess is that a long time ago some single event of abuse set things into motion, and the rest happened slowly over a long time. Sense it happened so slowly and there was so little metal left holding it together at the time of failure, I would rule out a design or manufacturer flaw. The design was at least 4 times stronger than the final failure put to it. Most likely 5 or 6 times stronger maybe more. This based on a small sample of total area of failure, so don't rely on this as gospel.

I also agree that 800 Hp out of 354 CID NA engine is doubtful, even with 12:1 compression. Maybe 354 is a typo and it's a 534 stroked big block.