I think the most likely place that J7 was mean't to be was at the point where it crosses over the A673 in Anderton.
http://maps.google.co...ited+Kingdom&t=h&z=17
The bridge that carries the M61 over the A673 is quite large and it looks like there may have been ideas about dualing the A673 at one time as there is space for a 2nd carriageway to go under the motorway. Therefore this would seem a logical place to site J7.
I have no idea if this was the planned site, just my speculation on the subject.