2nd gen is also a unit body. The subframe just supports the front suspension. It's supported by the unit body, the unit body is not supported by the subframe. It's short! The body structure, of which the roof is a big part (think roof trusses, like in a house -- would you cut the braces out of one and expect it to do the job?). Welding the "subframes" together won't be enough (the rear rails are directly part of the unit body). You'll need to build more frame into the car, or use a six point roll bar/cage. Another idea, since you're making a little roadster, is to weld the doors shut with a little extra bracing inside. You're taking the roof off anyway, may as well make it a climb over! That alone will add a substantial amount of rigidity. I plan on doing this to a car -- welded shut doors and a roll bar with six point mounting -- but the unitbody I'm using has continuous rails front to rear anyway.
I've driven plenty of cars welded together as you mention, even a 2nd gen T/A. If done correctly it's not a problem. Apparently there were a lot of hack jobs done in the UK so they made it illegal. Smaller cars aren't as sturdy as bigger US makes either, so it would be harder to do with the smaller ones. You need a good level floor to work from for measuring as well. Cut the windshield posts in the center on both. You may need to put some reinforcement inside the post also, and in the rocker panels as that is the main longitudinal support area.