I did the 2 smoke conversions at Dunsfold in the mid 80's.
Cutting the 3 large holes in frame 34 took a little moral courage because the modified tailcones did not arrive until just before we delivered the a/c.
There were no jigs etc at dunsfold as istr that the original smoke conversions were done at Bitteswell,so it was a case of careful measurement and home made tooling !
Luckily the tailcones lined up very well
The conversion is relatively straight forward,but as others have alluded to - some of the components are 'long lead' items and do take a while to manufacture.
I was not allowed to attend the engine runs on the 1st a/c,and a large cloud of smoke was followed by a van racing over to me to ask how to switch the smoke 'off'
I laughed and said ''doesnt matter - it'll run out in a few seconds'' LOL