What I did when flexplate bolt rounded off during my 4 speed conversion:

Took one of the bolts that came out, went to lowes/ace/homedepot/yourfavoriteboltstore, found a giant nut that fit snug over the head, plainest steel I could find, not galvanized or anything.

bought 4 or 5 of em (cost a few bucks)

Stick the nut on the rounded off bolt, weld it onto the bolt, start with some gentle tacks and try an impact on it, then work up to filling the inside of the nut with a nice solid bead if it doesn't come out before the nut snaps the weld off (thats why you bought more than one nut.)

Believe it took me 1 or 2 tries with a little 110v flux core welder before it spun right out.

The nut protects the flexplate from getting hit with a grinder or weld spatter or anything.

Used the same technique to remove the manifold studs from my f150. the nuts are normally 13mm, I could fit a 10mm on them. they all rounded off. In this case i was replacing the manifold so I cut the nuts off, then welded new nuts onto the studs and removed nearly all of them by hand. two or three needed a ratchet and socket, and one needed an impact.

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1982 300D (w123, "Grey Car")
1982 300D (w123, "Blue Car")
2001 Ford F150 "Clifford" (The Big Red Truck)
1997 Dodge Ram 2500 12V Cummins
1996 Dodge Ram 2500 12V Cummins
Previous Vehicles:
1995 E300D, 1980 300SD, 1992 Buick Century, 2005 Saturn Ion