Thanks, Mattax. If I can't find the stubs, I think I have enough room for a 3/8" thick adapter plate and a 3/8" thick header plate. We've done a couple sets like this in the past for RB engines that worked well. I'll make the adapter plates pretty much to port shape, and counter-sink the bolt holes to use flat head allen bolts to attach them to the heads. We shape the header plate ports to transition to a fairly round tube, and can position the retaining bolts in a convenient location. Looking at 1 3/4" at the head port and stepping to 1 7/8" about a foot out.

I probably have another couple weeks before I get the car to my shop to find stubs, and then I will be able to make the decision on what direction to take.