The key to improving cylinder fill average is to use three smaller carbs, and solid linkage, rather than progressive (or with the ends blocked off).
Anyone ever see a 3.8 Jaguar with progressive linkage, or with fake end carbs? Its been awhile, but I seem to remember they ran pretty well.
The 3.8 is about 232 CID, the later 4.2 about 256 CID.
As to the clearance issues, I don't know.
Jon.
