My experience with spring retained exhausts is all with 2 strokes, 350 TSS Bultaco and TD3 250cc and TZ350 Yamsels. If I can remember correctly the expansion chamber fitted over about a 30/40mm long stub either cast into the barrel or, bolted onto a flange.
On the Bultaco when you revved it you could watch the header/expansion chamber pushing out about 10mm from the barrel against the retention springs. No gasket was used and it never seemed to leak exhaust gas. The one piece expansion chamber /header pipe was a good sliding fit to the barrel stub but I suspect the exhaust was really located/held in place by the Mini exhaust isolation mount under the foot rest.
On the TD/TZs similarly no gaskets used and again no issues with leakage. Longevity was not a concern as the maintenance schedule if you wanted the bike to stay competitive was:
150 miles change piston rings
300 miles change pistons, rings and small end bearings
450 miles change piston rings
600 miles change barrels, pistons rings and small end bearings
Repeat sequence every 150 miles until
1200 miles change crankshaft.
On my Monster I have a fabricated (MADASL) SS exhaust retained by springs into the exhaust ports in the head, the exhaust flange is about 8mm SS and is a snug but sliding fit over the 1mm wall SS tubular header. I think it is much more likely to leak exhaust gas because the pipe does not fit over a stub if the retention spring is deflected there will be a gap between the aluminium port and the SS header pipe. Mark Lumb (MADASL) did not recommend that gaskets should be used but I, like you, worry a bit about the end of the header pipe over time fretting the step, cast or machined (?) into the head.
If you want /need to make up your own can try these people
http://www.pjengineering.co.uk/categories.php?cat=9