90 manifolds - modified? Pics inside- need to mate to Flowmaster Exhaust

[Corvette95]

So starting a new thread as I posted this in my other long thread I have going but not getting much attention. So unexpectedly, I had to pull my stock headers and Flowmaster cat back system to replace my motor mount and C-Beam plates.. I am going to bite the bullet and replace the headers with OBX because that was a planned future upgrade but the stock ones being such a PITA, I have them ordered now, can't spend the money on the Corsa catback that I want right now, so I need to mate the OBX's to the old Flowmaster, seems there is an issue on the right side, a prior modification? What do I need to do to get the two connected, car is up on stands and really can't get it to a trans shop, help and part ideas would be appreciated. Also, do I reuse the OE header stud that does into the oil gallery (resealing of course) I have new header bolts ordered. (seems the right exhaust has a flange added?) Thanks.