Stock subframe connectors are really just for show purposes. When I went to the MM full length SFCs, I was surprised by how much tighter the car was, it was a night and day difference. Took out some little squeaks and other noises also.
I have been told that a good muffler shop can weld them in, but it can be done by any competent welder with mig welding equipment. The car needs to be supported with its weight on the suspension and completely level, if not, then the subframes will keep the chassis twisted in whatever way it is twisted when the SFCs are welded in. I wouldn't pay more than $100 or so if I was going to pay someone to install them, though.