You have me thoroughly confused. The drain cannot go down the standpipe, and then up 7', unless the standpipe end is higher than that. In any case, having 7' of water in the pipe when the washer starts to drain will be a positive way to have it overflow the standpipe. Without seeing the current installation, we cannot even begin to tell you if there is a "safe" way, (i.e., without overflowing and flooding the basement), to connect to that existing piping. The only sure thing is that if you connect to the cast iron pipe going into the floor you will need more help than we can give over the Internet.