I know the 351 is a more robust block, but I think it's just too tall. Click on the link (not the pic) and you'll see a well known and successful racing, 351 equipped Mk IV. I think you can see anti chafe rubber on top of the air cleaner and the
oil pan dragging along the ground almost.
1987 AC Cobra MK4 6 litre 351ci For Sale on Car And Classic UK [C625850]
I went to see an FE equipped Cobra that doesnt overheat and the footwells are 29" apart. I measured my car up again tonight and that is an issue. Each footwell needs narrowing 1.5 - 2", which conflicts with the existing steering column and requires a pedal box mod which is no big issue.
The factory did it seems fit the FE in between unmodified footwells. How every everything is virtually touching as you can see from the pic of this, one of the 11 factory 427 BB equipped MK IVs.