I spent the last hour or two going through your entire 34 pages of board, as what you are doing is so similar to what I want to do with my 71 Cuda.
I learned a tremendous amount, so its was time well spent. I am also going with a 572, and an AlterKation (I live only a half our from their shop, in PA).
I was initially thinking Tremec, but now I want to investigate the Passion 5speed. That engine,trans, header setup sitting on the K was quit a thing to see, and I hope you don't mind, I made a folder and copied those pics for reference. The valve covers with the Huge 572 are perfect. And its great to hear and note what parts suppliers treated you well.
Getting to the end, and seeing your motor issues is like a good movie that draws you in and then has a horror twist at the end......Cant wait for you to update and give us the happy ending!
-Bob