[urq] subframe mount bolts

[SpotatAshleys]

Evening All

I have not posted on here for a long while due to a house move and having no computer set up until now.
I've just finished quick change of starter motor (old one - only 2 yrs old after-market, non-Bosch rubbish - fell to bits) and whilst having a check around underneath found that the rearmost left subframe mounting bolt (nearest to where the exhaust exits) has stripped it's thread in the little stalactite-like "turret".
My question is:
As the thread has stripped rather than the captive nut becoming detached and spinning around, is it possible to re-cut a slightly larger thread and use a slightly larger bolt to suit? If so has anyone done this and again, if so, what thread/bolt size was used?

I guess if the above is not possible then there is no alternative to opening one side of the "turret" and somehow securing a new bolt in there of the correct size and then sealing it up.
If the latter route has to be taken is it OK welding down there given the proximity of fuel lines etc or do I need to remove the tank etc?

Are there any alternatives to the above?

Look forward to any suggestions you may have.
Many thanks

Keith

'87 WR (in rare and unpopular Nile Green but I love it)