A few thoughts...
Would they have bolted on a HT back in the 50's had they been available? Freakin' right they woulda! Is it "tacky" and out of place like a spoiler & ground effects on a 58 Chevy pick-up? No - with a lil touch here and there the HT will look... fitting and right (if not period). Are the mods damaging the bike in any way? Actually, no - the welds are far superior than clamps in the long run and in fact proper welds can be undone as there shouldn't be any undercut or damage to the integrity of the tubing.
If say in another 50 years some fanatic saz "OMG what were they thinkin'?" it's a simple matter to cut the mounts off, grind flush, and build up new steel by welding and grinding if necessary.
If the back had been hacked off and chucked in the recycle bin and a modern rear suspension was JBwelded in it's place - now
that might give a purist somethin' to whine about lol
I see nothing here but respect for a great old bike that's gonna get a second chance at life doin' what it was meant to do - bring a smile to yer face as you blast around the block feelin' like the kid we all still are