[Tom Goldman]

Spend the time you would spend on welding the seams, not recomended, and use it to put in a complete roll cage rather than a roll bar.
A 4 speed car will thank you for the extra stiffness the cage will provide.
I would add a bar from the cage to the firewall to stiffen the front clip mount area.
You can also add a brace from the cage bars to the area Danny mentioned to stiffen the clutch/brake mounting.