Better to use stainless but dont put to much heat on the stainless bolts or they will draw up when they cool and you will never get a nut on them.

i can't really offer too much except an opinion, i guess all of us do that. a rule of thumb has always weld stainless to carbon but never carbon to stainless.

in the case of your vessel, cutting the olet (socket) off was probably a good move, though i wouldn't have cut a big hole and replaced it with a bigger olet but it was probably a thin wall tank and there was no other choice. if the tank wasn't pressurized it probably would not have been a problem. although if it was subjected to vibration that could be a problem.

the main question is, was there a qualified welding procedure available for the installation? if there was and the procedure had met the requirements of the code or standard, there shouldn't have been any problem, right? show me the qualified procedure and we all walk away happy.

Shane, it's too bad you were subjected to ridicule.  Ultimately, I'm sure the part would have failed prematurely, although agree with your stance that it would have been fit for service, perhaps adding it's removal to some future scheduled maintenance task.  As you know, the welding of stainless with a carbon filler is a definate no-no but then again, in the limitied risk application coupled with the protective coatings, it probably would have lasted a good long time.