Basically the longer the flex joint the more movement it can withstand without failure.
Also the less movement is transfered to the exhaust manifold studs (i.e. broken studs)
Why is that? Because the exhaust is mounted in a fixed position so something has to give when the engine rocks back & forth. (and boy does it ever) The exhaust mounts have very little movement so if the flex joint can not compensate then another point will.
I chose 8" flex joints; that gives about a 5" actual flex piece. Larger then the tiny ones Brad used. Those are the smallest size. 5" or 6" IIRC. However he had solid engine mounts to so he could probably get away with them. Lucky so-and-so...
