Welcome Mitchel!
Nice looking car of yours!
What has broken on your clamp?
Several members at 300 Power Club has faced the same thing as you have.
But I have not seen another solution than exchanging the broken coupling. Not other than rebuild the car for a front mounted gearbox + propshaft + stiff rear axle.
But I suppose that you search for a "easy" solution?
An earlier member made the coupling on the torque tube a bit stronger with help of a lathed steel tube that was made so it fitted perfectly on the original coupling BETWEEN the two clamps on the coupling. I have not seen an image but think it would look like on the picture I added to this answer.
To fit the strengthening, one clamp must be removed and then refitted with the welder. I suppose..
Greetings
Frej