Bart... I'm fighting the same fight on a -7 with SJ cowl and IO-375 right now. Was the consensus of your talk with Van's and others that 1/4" is enough clearance at the rockers? I have maybe 3/8" at #2 with hinges at present.
Also - after perusing some Lancair builder's sites... I see many elect to use the fiberglass and screw option for clearance issues on the side parting lines. After looking at their sites I didn't feel so bad about my clearances in the engine room
i would think there would be advantages to containing air leakage at the sides for cooling air drag reduction also with this method. Ah, the joys of cowling mania!