If you don't fix it, 10 years from now you'll look back and still remember it fondly and wonder WHY? (I really miss my 72 dart from back in high school

)
Like others have said, take a good hard look at the rest of the underside. 700 is cheap if that's all it takes to make it solid. Get some ramps and pick at the rust and old undercoating to see if the floorboards rockers and torsion bar mounts are solid. If you get the back part fixed, you should consider coating the rest of the underside with something to curb future rust till you can do a real restoration. Also check your brake lines back there.

They are pretty important too.
I once fabricated new rocker panels for a truck using some heavy steel shelving found in a scrap pile. A couple of 2x4s (as forms or bending tools), a hammer , a sawsall, and lots of free time, but it was rock solid when I was done. Take note that one angled piece is more rigid than many flat patches.
Welding is preferred but not required. Make your repair panels big enough to extend to solid places. Self-tapping metal screws can then be applied in abundance. (These are great for "temporary" repairs

). Now you can drill and bolt new sping mounts to your "frame".
You should easily see about now how this is $700 worth of work. Dodges are "unibody" and require labor intensive repairs to correct "frame" problems. Most shops around here won't even touch a car that old with rust issues, so that mechanic must like you (or your car).
Sorry for posting as a guest, but I can't seem to get a registration working (A know issue with my email

).