Donn, here's a visual which might help - 351, FE and Ford modular. I'm no expert but I would guess the SOHC 427 cammer is a tad bigger in terms of breadth and height than the FE, but smaller than the modular. Again, I don't really know - there are lots of folks here who would know however.
Sounds like there are a lot fo Roush 427s in SPF cars, so, I'd guess that the main limit (if there is one) is the additional height related to the separate cam and drive, and whether that all fits under the internal horizontal part of the clam shell....
Would be pretty cool to see a "cammer" in a GT40.