Like pretty much any engine on Drag Week, most spare parts center around the valvetrain, like rockers, pushrods, springs, and in the case of the FE, the shafts (on stock type valvetrains). I carry spare shafts, rockers and pushrods, but have never needed to do anything to the engine in the 2 times that I took the Mustang. Your engine is a whole different animal though, so I think I'd have a spare for everything in the valvetrain, except for the cam of course.
I also take a few pieces that I know you can't readily get in parts stores anymore, at least not immediately. I take an alternator, distributor and water pump. In my thinking, it's easier to take the spares, which I already have, than to pay thousands of dollars to have something overnighted. Ok, maybe not thousands, but it ain't cheap! Any more than that and I figure I'll be done, because I'm not one of those guys who will thrash to rebuild an entire engine in one night...on the side of the road.
Transmissions are another problem area for a lot of cars. My car isn't really powerful enough to break what I have, but your twin turbo setup may be another story.