IMHO, the only frame connector "kit" I've seen that makes any sense to use is the US Car Tool connector. It's welded to the front and rear subframes and to the contour of the floor. The only other good way to do it is to cut into the floor and weld the rectangular tubing where it passes through the floor. I think those are the only real ways to do it where you add strength to the car.
There's a lot of other ways of doing it, but IMHO, they don't do much to help. If you really need to strengthen a car, best to look at multi point roll cages.
Unless you are building a high HP/Torque engine.... Spend the time and $ somewhere else.