My vote is for missing dowel pins allowing the tranmission to rotate relative to the block under power. The bolts which hold the transmission to the block using compression are not designed to prevent the two rotating against each other under torque. In fact, the bolts fit so losely through the transmission side holes before engaging in the engine side threads that I wouldn't put it past the engineers to have designed the part such that the shank of the bolt doesn't touch the inside of the bolt holes on the bellhousing. The dowel pins are designed to be nearly press-fit which virtually eliminates torque hammer between the two parts.

I have seen similar failures on heavy machinery due to an under torqued condition. It allows a shock load to break a bolt in a way that hundreds of pounds of torquing could not accomplish. But I could be wrong in this instance. All I can do is speculate.

After a quick flip through your build page, it looks like you've got a very stiff chassis with very little flexibility. A system can be over-restrained to the point that it becomes 'infinitely rigid' compared to the components it connects to. When this happens, loads get redistributed to the least rigid components and it leads to failure. I'd like to say that you're chassis is so stiff that the lack of chassis/ppf flex put excessive tension on the bolts and failed them.

Any update to this #Laur3ns? I know this is an old tread but what did you end up finding out? and doing about it? I have the same issue

Dowel pins were not missing of course. Some 12.9 replacement bolts have taken care of this issue.

There is a guy in the UK who went through the same story and it worked for him too. Quaife apparently now includes better bolts in their gearset.