I've looked at BSA/Triumph 7" and 8" brakes, John. It would seem that a pair of fork legs (or just the sliders?) with bolt-on end caps would be necessary for the spindle - or have people who have used these brakes made a spindle which passes-through, like the original Ariel one? If so, what about the torque stay?
When I was building my Square Four between '68 and '70 I wanted a larger TLS brake so went for a BSA A10 front end in the Ariel yokes.
The Ariel yokes are wider than the BSA ones and I had a local Engineering Shop turn-up a longer wheel spindle. This left the torque stop (its a lump on the fork leg) too short so a Blacksmith friend brazed an extension on it (!). I was a bit concerned about the added leverage on the fitting but I'm still here and it never failed despite some wheel-locking stops (I was 23 back then remember).
I bought a 'Pride & Clarke' TLS brake which completed the conversion.
I'm older & wiser now but it all worked very well (I used to ride it hard - the bike was only 15 years old at the time so not an 'Old Classic' as they are today, and spares were available).
My avatar shows the late Len Rich in 1970 with the bike I now have - a 1958 Ariel VH