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You probably want to start by installing solid sub frame mounts. I don't think the connectors will do much if the sub frame is flexing around.
Yes they did. I had a 78 with a 400 and it was a fun ride. The previous owner put an 8-3/4 with a 3.91 sure grip in it and it would move out.
I still have all my 1978 Magnum parts...I still have the dash and power seat. Email me if you still need them.