I would agree with your imagination that the tension in the exhaust system is not good. Somewhere in that system is an area that's absorbing the brunt of the vibration in the exhaust system. That area will work-harden and will eventually crack, just like when you keep bending a paper clip at the same point.
The manufacturers mitigate vibration stresses with rubber-isolated hangers, flexible exhaust sections, ball/socket joints, and/or slip fits between pipes. That's why you rarely, if ever, see a factory exhaust system that's fully welded from manifold to tip. There's always at least one slip fit in there somewhere, usually at the mufflers. These days a braided-steel flex section is fairly common, especially in transverse-engine applications where the main vibration source (the engine) is at a right angle to the exhaust system.
Obviously, hundreds if not thousands of shops have been fully welding systems for years and never had an issue with doing so. Ultimately I think it's more a personal choice than a real durability concern, especially on limited-use cars like ours. Ain't no shame in clamps; they've worked well for 100+ years.
But if your system is fully welded, you have no flexible point (like a slip fit) where the pipes can be wrangled or manipulated in relation to each other to improve the fit. If the angle of the engine/trans changed, then you put the exhaust under strain. That will lead to a crack eventually.
When you massage your H-pipe to fit the new headers, make sure it's an easy fit to the new headers while firmly affixed to the rest of the exhaust. There should be no struggles, stretching, or forcing anything into place to finish the job. It should just lift into place and align, with the collector bolts slipping into place easily. If it doesn't, find the bind or pinch and correct it... that might require a cut. Once everything is bolted up to your satisfaction, welding it solid is your option.
When I did exhaust on my beater A-body this summer, I made the X-pipe solid almost to the mufflers. I used band clamps ("this one time, at band clamp...") to connect to the mufflers, and regular U-bolt clamps after the mufflers. I did that mostly to enable replacing the mufflers if I didn't like them, but if something else changes up front I still have some ability to jockey things between the headers and tailpipes and get everything realigned.
I also used band clamps after the axle at the resonators--yes, I managed to squeeze actual resonators into an A-body--but the tailpipe turndowns are welded directly to the resonators.
I had intended to use ball/socket joints designed for header collectors, but the way the TTi's flanges are welded that arrangement simply won't work. I went the old-fashioned route insteat. I did use Percy's aluminum collector gaskets, though.
So far, so good... and quiet, like a proper sleeper should be.
