If you've a MIG with a narrow head you can put a good sized spot weld to secure the captive nut without removing the subframe through the drain hole adjacent to the captive nut
You have to be accurate though as you don't want to weld the bolt to it, tricky as you have to sight it then weld it blind, certainly not for the faint hearted
Have done this in the past saves having to drop the entire subframe
Russ