Yeah i had the same problem too. i believe that it important to have all rubbers in place as the sierra body moves a lot and can crack if unsupported.
this is from when i asked viewtopic.php?f=5&t=15347&hilit=+body+mounts
(How do you make the clicky blue text? the globe icon is gone)
Ok, so this is kinda how i did mine. All mounts were attached with sikaflex to the chassis.
* black circles are thick square ones.
The front 2 mounts are 15mm apox square rubber mounts that are bolted down.
The diagram should help with the bulk of the rest.
however the rear 2 mounts i used circle rubber mounts, bout 5 mm thick , from memory.
Basically if you put the 8 main ( front, rear, 6 in the guts) mounts in, then stuff whatever fits into the remaining spots.