I would look at the chev metric chassis..that is 70's monte carlo..
the ford ltd is an option as well..
Later caprice chassis is also usable..
Morison and Jegs have tube frame kits that may be an option as well..
In any case I would be looking for a complete donor car..including the engine and tranny..
Measure the width of the chassis to see that the rambler body will be wider that the donor chassis..
You may have to channel the body to get the body height down to where you want it to be..in fact I would expect to have to do this..once you have the chassis prepped then you will see just where you have to cut when you set the body down..
You may wish to keep the donor body around as it may be possible to cut the floor pan out of the donor body and adapt it into the Rambler Body..
This seems to me to be a fun kinda project..keep ya occupied for a while..