Go down to post #3 in this thread. You will see how that bolt looks, and get an idea of what you need to do to remove it.
https://www.britishcarforum.com/bcf/showthread.php?108693-Recipe-for-a-TR2
The bolt will not turn, as it is keyed into the frame. It just pulls straight out. It is also highly unlikely that it will be able to pull out at all until it soaks with PB blaster for several days. Worst of all, if the broken bolt cannot be removed to use all-thread and a nut (not another bolt), then the only recourse is to raise the body off the frame enough to hammer the end of the bolt inward...or just remove the spring with the body up allowing access.
Once you get the bolt out, screw in a length of hardened all-thread. Place a 1/2” drive deep socket over the head of the bolt, so the socket rests against the frame, entirely covering the round bolt head. Then add a washer and nut. As you turn the nut, the all-thread will pull the bolt out, using the socket as leverage. Once the head of the bolt bottoms in the socket, you will have to replace the socket with a longer pipe, tube, etc to keep working the bolt on out.
It is very common to have the bolt seized so tightly that lifting the body is the only way to remove it.
I notice you tend to jump into some of these little projects without studying the manual. If you post your plans ahead of time, we can give you a heads up on potential problems. The TR3 is super simple, but certain jobs...like this particular bolt...have fretted even the the best of mechanics for over 60 years. Like mentioned above, many have gone as far as drilling holes through their body just to remove the front of the spring.