i just did this a few weeks ago, and it was a bear getting them
in. is it going in at all with the jack and then popping back out
when you release the jack? i ended up jacking them in and then
let the thing just sit for 15 minutes or so; eventually it held about
1/3 to 1/2 of the way. then i jacked in the large bolt (through the
spider plate but without the small bolts fastened) and used an
open end wrench to get the large bolt to thread. once it took,
then i threaded the small bolts; repositioned the jack to put pressure back on the mount and tightened up.

hope this helps. good luck.

__________________
rayroyw
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'85 380SE (238,300) my "new" old car!
'84 300D (303,000+)
'72 250/8 (mercy heaven)