You probably already know this but I have personally seen two types of CI Bellhousing for the 5 bolt pattern. Mine says "HRA" on it. I assume this pre dates "NHRA", was before I was born... I have also seen the COBRA bellhousing which is marked across the top just like the 6Bolt CI Cobra Bellhousing. Bob Shaw Has or Had(I assume he still has it) a Unique I looked at years ago when i was trying to decide which car to build. His had the 5Bolt Cobra Bellhousing. It's a long shot, but you might get in contact with him to find out if he still has the car or maybe knows of another bellhousing.
Performance & appearance products for Cobra enthusiasts