I'm replacing the header pipe on my '88 244 DL.

I have replaced the header pipe two times previously. Both times the manifold studs at the header pipe came out with the nuts. So I purchased new manifold studs from IPD but they turn out to be for the valve head not the head pipe.

I have found that whenever possible to replace nuts with stainless steel threaded together with anti-sieze. The SS nuts and bolts from the header pipe to cat came out nicely.

The manifold stud nuts took some wrestling to get off with PB Blaster and time. This time the studs did not come out.

I took the old nuts that secure the header pipe to exhaust manifold studs to the hardware store and they match 3/8-16 thread.
I buy SS 3/8-16 thread.

I get under the car and the new SS bolts will not thread onto the studs.

WTF?

I wire brush and wipe down with brake cleaner the studs. That does not help.
I find even the old nuts do not thread on easily but will thread on if I use a socket.
Is this wear and tear on the old studs AND nuts that have changed the profile of the threads?
Am I now committed to changing out the studs?

I went back to the hardware store and checked the original nuts on all the closest metric bolts and nada. The original nut still only threads onto the 3/8-16 bolt.
I double checked all the new SS nuts and they were the same 3/8-16 thread.

In addition, and this does not impact getting the new SS nuts threaded on but, staring at the studs I see the the threaded diameter gets smaller before it reaches the manifold flange.
No I am not seeing the blank knuckle stop.
It is as if the threads go from 3/8-16 down to something else.
Could that happen if someone tightened down with a coarse threaded nut over a fine threaded stud. Or is this OEM?
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1988 244 DL; B230F; LH-2.2; Manual 5-speed (M47)