Ok, well with what Mr Speedy mentions about the fuelling leaning off, I'd lay a firm bet there's detonation/knock at high rpm/load, and it's that, coupled with the high rpm, that's causing these rod failures.
Likewise, if bottom-end lubrication isn't sufficient enough (hence asking about the bearings condition), that would naturally induce extra heat into the rods, and with the additional/varying heat cycles, that may possibly also weaken the rods.
![Agree](images/new-smilies/agree.gif)
See what you can find out about the lubrication, and if all is good, and if you can keep the fuelling happy at the top end + don't be too rev-happy, I reckon you'd dispel that 300 lb-ft myth...