attaching point

I decided I better weld in my subframes before I hook up my traction bars on my '64 merc. I fabbed some up and have the front attachment worked out but what about the rear? My rear subframe comes down and the front of the leaf attaches to it, then turns into half a frame with floor attached to one side (towards the outside body) but the inside edge still open. Can I still weld them to the 1/2 fram section or di I need to go through the floor and attach to where its still a full boxed frame?
Thanks
Tracy