I know that not a lot of people are doing this swap anymore but some have the swap done, or are in the process of it (like I am). I have been working with a machine shop trying to make the front manifold of the 96-97 3.4 dohc motor work on the back head. most people know that the 91-95 swaps you can just switch them but the 96-97 one if you flip them the exhaust port in the head doesn't line up. well I have made up an adapter out of 1/2" steel and will be in the process of getting it cut and mocked up. when I work out all the kinks of this working, I will be putting them up for sale for no profit and I will release the cadd file for whoever wants to use another shop to cut it. I will be posting pics and information as I go through mock testing and if I have to fixing the cadd drawing.

I might be down for a set..

Yes a large radius fillet will reduce the "flat" for the bolt head, but if these mounting feet are cut with a large radius fillet along these edges, they could then be finished with a flat faced end mill to provide the full diameter flat surface for the bolt head.

The large radius fillet will reduce stress risers that can lead to cracking.

I wouldn't even machine that surface shorter than the rest of the part. That way the adapters can be cut via water jet or laser at a much lower cost. Some laser beds also have built in drilling/tapping operations, so a single setup could do everything on the bracket. Then if you wanted to get fancy, you could counter bore the holes for the bolts that attach the bracket to the head.