Chris,
Laurie Bongailas (RCM) is developing his chassis to accomodate a 5.4 Boss. If you want to give him a call let me know and I'll send you a PM.
That said a 5.4 is full of compromises -
For a start it is 3 feet wide so it is unlikely three pedals will fit or if they do they will be serverely offset. The intake manifold is too high, however this can be solved by fitting a custom lower manifold, being developed as we speak. Clearence problems with the steering shaft, modified
oil sump, inspection holes in footwells for exhaust manifold bolts etc etc. These are the known problems at the moment - all are apparently fixable.
Or you can just join the dark side and drop in a Gen 3
Considering a Gen3 makes as much power, is much easier to install & a weights A LOT less, it is not surprising that many new builds are using them.
Cheers