Another thing are all the brackets in the engin compartment for the air canister and other junk. buy a drill to remove them (spot weld drill?). Depending which year you have some of the bumper mountint points were much beefer and an area to look at removing/lightning. Ron was right about the dash the 280s were a lot heaver than the 240s if possible pull it and gut it they had a lot more material and braces inside and if the dash is out remove some of the mounting brackets for the ac and other interior parts you are removing (drill again). And like everyone said the undercoating. When we built our IT 240 we weighed the undercoating from the floor pans and interior got 12lbs out.
Yikes, be careful how far you go w/ removing things that require the spot weld drill to remove. The emissions stuff can go, but its mount is not listed as optional. For example the fog lights mount to the bumper structure, removing the lights is OK, but taking the plasma cutter to the structure it was mounted to is not.

Do the 240's and 280's share the same PN for the dash?

Being overweight sucks. Still gets under my skin that the great re-alignment couldn't have bumped the 240 & the RX7 up 50# to give cars like the 280 a fighting chance to make weight.