There are weld-in and bolt-in connectors available. Some can be installed either way.. I have some from TCI but they are not usually sold independently of their suspension systems. They are designed to be bolted in but can be welded in. They are an easy fit and have no floor interference issues. Global West makes a weld-in connectors that are a good fitting round tube design that clears all floor contours.. Both of these connectors also clear the parking brake cable allowing it to remain in its stock path.. Competition Engineering makes 2x2 square tube subframe connectors in two different designs bolt-in which can be welded in and a weld-in only connector that are meant to be welded in and connected to their parallel rear frame rails.. The CE bolt-in and weld-in both come in contact with the floor pan and will need to make modifications to the parking brake cable routing.. Detroit Speed Engineering has weld-in only that are very intrusive into the rear foot well floor are.. They do probably offer more chassis stiffing benefits since there is a lot more contact area to be welded too but the tubes will be up through the floor in a big way..
I know there are more connectors on the market but this is probably enough to give you a good idea of the basics of what’s out there.. Some are basically copies of the ones I discussed here..