Rob!
I note lots of problems with this area on the brickboard. I have never
encountered anything like it on any of my oldtimers over 35 years of driving
and working on them. I've always used standard 5/16"-18 UNC standard nuts on
my B18, B20 and B30 manifolds, RARELY replaced one unless it came off and
got lost, and never had one hard to remove, even when a stud was stretched.
The exhaust pipe flange nuts are special stainless steel items, 3/8"-16 UNC,
and likewise never gave problems other than occasionally coming off and
getting lost. So what is the problem with the newer models? I can see
some possibilities:

1. Finer #$%% metric threads
2. Smaller #$%% metric bolt sizes
3. Higher exhaust temperatures (which on an oldtimer would indicate
retarded ignition)
4. Inferior stud and/or nut metallurgy (I don't see how any copper alloy
would stand the required torque unless perhaps beryllium copper, which is
pretty rare these days)

I wonder if the same type of stainless nuts used on the old exhaust
flanges might be a good choice. I would imagine that they could be
found in #$%% metric sizes from some aircraft supplier, and if bought in
quantity, should not be very expensive.

I also wonder if the studs could be replaced with dual-thread studs to match
the threads in the head on one end and coarser threads on the other end
to reduce seizing opportunities.

Any comments you or others might have on this problem could potentially
be helpful to all those who use the newfangled OHC engines.

--
George Downs, The "original" Walrus3, Bartlesville, Oklahoma