It seems some of the bodyshells didn't have the nuts fixed to the "T" bar on the lower mounting points. I had this problem too and was advised to cut two large holes into the bulkhead and either fill them with fibreglass or fit speakers. (I'd already fixed the shell in place and tank etc.)
The only other way is to remove your tank.

You must use 7/16" UNF ON ALL SEATBELT mounts else you'll fail the SVA.

Cheers
Adrian:tu