I've never heard of this window weld you speak of, but it sounds ghetto rigged. I'd be afraid to screw with something as important as motor mounts. And depending on how bad yours are cracked or shredded, I'd buy them brand new. CAI's, MBC's and other things are fine to buy used. But motor mounts, internal engine parts and springs are things I would think twice about, because you just never know.
Just remember, the more mounts you install, the worse the ride quality in the steering wheel feels. Gets worse and pretty old when you drive for long periods of time. I wouldn't be able to stand a drive like that personally. But it may not bother other people, so I can only speak for myself.
