Sub Frame Connectors after Interior is finished

You're going to melt the sound deadening. Even small sections of weld will do that. If it's the butyl backed stuff, it gets soft and tacky at 100*. Not only will it melt, it will catch fire. The paint in stuff like lizard skin might fare a little better, but it's still going to melt and burn.

You're best bet would be going to a tubular frame connector, you can go as big as 3x1.5" without interfering with the floor, all of that welding would take place on the frame rails so the floor should be ok.

The Mopar bolt in connectors work ok if welded in, otherwise the bolts just back out after awhile from the flex. You could make bolt in's work but you'd have to do captured nuts in the frame and reinforcing plates at the connections. Easier to just weld them in.