Ppl have done it from below, after droppin engine as low as it comes. Manifold out, machining, all studs to be replaced with hi stregnth studs. If u keep it, do both manifolds

Heres one link, I suppose there are plenty of writeups as the issue is common
http://www.4dsc.com/articles/drivetrain/exhaust_manifold_studs/exhaust_manifold_
studs.shtm

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Wiikinki
www.cardomain.com/ride/748507/

I had this with two outer manifold bolts on the front cylinder head of
my V6, I found they had sheared just under the surface. I drilled them
out with a DRemmel and carbide burr in situ then piloted a fresh 8.2mm
hole before using a helicoil kit. It was very awkward and because the
pilot hole was free hand and the helicoil short I don't think I left
enough meat for the thread but as it feels "soft" as I tightened it.

The manifold seems to be a welded construction and there were other
bolt holes nearby so there may be scope for clamping off other points,
otherwise I think I would have the head off and do a better job on the
bench.

AJH

I've replaced a BUNCH of cylinder heads from people hitting the water jacket
trying to drill them out themselves as their first attempt at stud removal.
It's not easy doing this with the head installed for someone who has
removed thousands of broken studs. For a first timer, all I can say is be
careful and DON'T break an -easy off- out in the stud as you'll have to
replace the head if you do..

This job is a pita.

I replaced two broken studs in my 94 without taking the manifold out.
In my case it was possible because the gasket had not burned.

For the rear manifold, i removed the right hand side axle and used a
drill with a 90 degree attachment (small size one, bought on ebay for
$20).  Getting the center of the stud was not easy and took me at least
1/2 hour to get the right location.  The manifold holes for the studs
are not centered with respect to the stud so they cannot be used as a guide.

Once i got the center punched, I began drilling with a small drill,
making sure that the hole being drilled was aligned with the stud and
that the depth never exceeded the lenght of the thread.  I then used the
final diameter drill bit.  When that was done, i used a good quality
screw extractor and was able to take the broken pieces off.

Good luck!!