thank to everyone for the suggestions, hopefully its knowledge for all
1. followed the suggestion ( kato's?) to place a some rubber on the exhaust pipe where it meets the cross member. no difference, other than numbnuts did not remove at the end of the experiment and was then driving around with a burnt rubber smell. so dead end there
2. so took the car for the RWC and the guy found it.
ready?
ok,
1. here's the diff mount i got from zcardepot
https://zcardepot.com/catalog/product/view/id/336/s/differential-mount-insulator-oem-240z-260z-280z-71-78.html/
standard nissan issue
2. its not damaged in the car, just like i said
3. however, instead of sitting symmetrical on the cross member like this
http://woodworkerb.com/wp-content/uploads/2014/11/Figure-RA-3.png
its stretched tight on the front and compressed at the back to the point that the mount lips hits the cross member when the diff moves
4. i have the straight rear cross member like this
https://c1.staticflickr.com/1/407/19369581915_445221a6b8_o.jpg
5. stewart wilkins ( god bless that guy) from sw motorsports tells me that i need an EARLIER version of the mount - that no one makes anymore - so that the diff sits more forward
6. i think this is right because lots of the suppliers list a 72-78 mount and not 70-71
55415-E4102 - this is the early mount
55415-N4300/4301 - this is generic 280z one that i have
7. from what i can see, the difference seems to around 5mm between the two in regard to the bolt centres
so ........ question is:
A. have i got the right end of the stick
B. surely someone has done this before - is it a matter of ovaling the holes on the mount or just turning it around?