Update on my J4A - The local GADO office has been cooperative but it turns out there is a nasty AD on the Hanlon Wilson mufflers that I was planning to use. AD calls for 25 hr inspections for cracks - and no one is permitted to fix them. I decided to bite the bullet & now installing a C120/140/150 muffler system. Trouble is the nose bowl has some interferences so it is chop time to my cowl. But the muffler system is a much better package & I don't regret having to do it. The IA said that OK City FAA engineering would be very leary of any engine swap schemes that used that old muffler scheme.