I have had some "discussions" with MTO and individual mechanics doing inspections over the last 14 years (my time in the saddle).
It is my experience that as the "rules" are antiquated and overdue for re-write, the mechanics have a fair latitude, keeping in mind that their "ticket" depends on them not breaking the rules too much ( or getting caught for passing some truly dangerous).
Some guys are hard asses ( I had a guy fail me for the headlight not pointing in the right direction) but we all know someone who won't even look at the bike but write you the cert for some "readies" as the Limeys say. I have experiences with both and I prefer someone in the middle.
The real issue comes with the insurance companies and not the certifier, although the mechanic may indeed have his own interpretations.
Insurers, as you know, are a royal pain in our collectives asses.I would venture to say most would not entertain a modification of this time.Many do not approve of any modifications whatsoever. At the very least, I would expect them to require you to prove that the welding was done by a certified welder.
Many guys get caught out by the insurer if the bike was involved in an accident and they find out it was modified and they hadn't been advised. That is an automatic disqualification of coverage. I know of at least one serious situation where a guy did a 7/11 swap (1100 mill in a bike registered for a 750) he caused an accident, destroyed another vehicle and injured himself and others. Insurance reneged, he got sued and lost home, savings, family and his job. It wiped him out.
Insurance companies will take any avenue to get out of coverage so it is always best with custom situations to get approval "in writing" before doing much of anything. If you can't get insurance.....you can't ride on the road can you.
Good luck with the project. I hope you can get 'er done!