You are better off using box section for your frame connectors. That way you can easily weld them to each subframe and to the floor too at one or two points.
Some people are making connectors that are shaped to follow the shape of the floorpan so you can weld them along the whole length. This is ideal but not needed in most cases.
Connectors are easy to make yourself if you can weld. Mine are made of 2"x2" box section. They are not welded to the floor at all at the moment and have been working well as they are. However i will be welding them to the floor at one point at least sometime in the next few weeks.
I'm not sure what is worse, me posting while at work or you guys responding instead of sleeping!
It was only 9pm here when I posted my last message and it is coming up to 9am here at work now. Bloody time zones eh?