Tony, it's legal in V4. The answer I received was both informative, and helpful. We will continue to work on a design that will accomplish our goal, and be legal.
Here is the reply I received:
The Vintage Engine Committee has previously found the use of a plate to adapt a cylinder head is not an acceptable modification, except in V4. The Committee felt they did not want to stifle innovation, so any 2.A.1 compliant specialty head may be used but must bolt directly to the block using the original bolt/stud holes. The use of larger and additional head bolts/studs, as you recently requested and was approved, would continue to be acceptable.
It sounds like you have two issues that you are dealing with; providing a combustion chamber and attaching the cylinder head. One method to create a combustion chamber when one isn't located in the cylinder head that has been used in the past is to lower the piston in the cylinder. Another method that has also been used in the past is to weld a plate of similar material to the head thus making the plate an integral part of the head and retaining the original head bolt location.
A Specialty or after market head may be modified to use the original head bolt pattern in the block by welding up, and adding, head bolt holes. This is more easily done if the head in question is aluminum, but it can be done with cast iron as well.
Thank you for your interest in the Vintage Engine Classes.