You can install subframe connectors. Problem is that there aren't any for sale, so they are custom made. I am not sure if anyone here has done them on a wagon, I have a 74 wagon and want to do them as well.
If you installed the cage yourself I would imagine you have the skills and equipment to come up with the connectors based on what some have posted here, just have to do some adjusting for the wagon specifics. Me, I don't have those skills or equipment, so I am not sure what I will be doing. Might have to make some out of wood and take them to a welder and say please make these in metal.
Maybe also leave the forward part of the cage, if you have it running from the firewall to the front frame rails, or if not then add them there?
Russ