Smooth transfer of power

With 5000 lbs of force pushing on the body of the rocket engine, you have to bolt it down securely or else you'll have a 300 lbs steel tank flying through your windshield and the car sitting there steaming.

Since a roll cage is really a necessity in a rocket car anyway, I figure that's really the best way to transfer that enormous amount of force to the chassis of the car. I haven't been able to find any exact dimensions of real 924 or 944 roll cages, but I've eyeballed some pictures on flickr and I think I've built a fairly accurate model. Everything is parametric anyway, so I should be able to change it around once I find some real dimensions.















Also, I came across a possible difficulty in manufacturing the design as it stands now.
The problem is that the exit diameter of the nozzle is larger than the bolt hole circle that connects it to the rocket. It's clearly possible to drill the holes for it from the far side, but all of these holes require counterboring (or spotfacing) as well:















My first thought was to use a normal counterbore, pushing the shank up through the hole and into the spindle of a mill, then cutting the spotface by pulling the counterbore upwards. Unfortunately, no counterbore I've ever seen has a ground cutting edge on the top. Also, very few have a shaft that would even fit through the M8 bolt holes.

So what I think I'm going to do is use a 5/16" boring bar (McMaster #32435A12) with a tiny nib of a cutting tool sticking out. It is going to be a pain in the ass to insert and remove to tool 20 times though.