An alternative solution would be to carefully measure and remeasure the bolt locations. Drill down from the hatch area using a hole saw to expose the studs. The hole saw should be big enough the let a deepwell socket reach through. Install a nut on all studs. Weld these to the studs on top. Soak these with Kroil or PB Blaster over night. Put an impact or breaker bar on these and turn them out. Run a tap from the bottom to clean the threads and use rubber plugs to seal the holes you drilled for access. Be sure to wear goggles! I've had rusty metal removed through foolish behavior...it isn't fun.

1. Take a cutoff wheel and cut the stud flush with the subframe.

2. Center punch the exact center of the stud.

3. Using a left hand 3/8" drill bit drill out the stud using a lot of thread cutting oil.

4. Once the stud is drilled out get a 10mm x 1.25p nut and measure the OD at the points.

5. Get a drill bit or a Unibit that exact size and enlarge the 4 holes in the subframe.

6. Clean up the surface of the subframe around those holes.

7. Weld 4 of M10 x 1.25p nuts to M12 washers.

8. Push the nut side of the washer/nut combo into the holes you drilled in the subframe.

9. Weld the edge of the washers to the subframe.

10. Grind flat and paint.

I have had success in using a high-strength alloy electrode in welding out broken studs and bolts. The probable reason why your friend's wire welder wasn't successful is that he was likely using a low-tensile wire, while the bolts are considerably stronger, say Grade 5 or better. Mild steel wires/electrodes aren't generally successful when welding higher grade steels.

Use the smallest diameter electrode you can find, 3/32" at the largest, E 312 or E 309(L). It sounds like you have the rest of the procedure down pat.

John's suggestion above is also a good one.

Thanks all, I am nearly done. I tried so many iterations of welding to the studs but they would not budge. I decided to drill out from the top so I carefully cut into the deck floor to expose the studs from the top using a jigsaw and careful "plotting". They were fusion rusted!

I have drilled out 2 studs to 3/8" from above using lots of cutting oil. 2 more to go then I will follow John C's advice.

I sort of used all the advice from all of you! BIG THANKS.

I'll post some photos soon.

BTW this 71 Z was winter driven here in Atlantic Canada for its first 5 years. This area is proving grounds for rust protection due to road salt, salt sea spray, salt fog, constant rain and mist, winters ~ freezing with nearly daily melting and thawing of road moisture and salt. Sadly a rusted stuck bolt here is a very common occurrence.