The header choices for the 351-based Ford Boss 427 crate engine with the raised-port heads are very limited. In fact, when we did this install, FPA was the only source we could find.
They are very pricey, but beautifully made. They fit perfectly on the heads, BUT fit in the engine compartment was something else entirely! The basic problem is that the FPA headers do not hug the engine quite as closely as they should.
To get them in, we needed to do basically 3 things:
1) Notch the passenger side frame rail.
2) Grind down the passenger side shock tower reinforcement.
3) Shim out the upper a-arm bolt on the driver side.