What I thought was a direct replacement wasn't... Help!

[84LX89GT]

chatcher is absolutely right, they are a DIRECT fit. I just cut off the sealant sealing the lens to the my original '84 lights and put some '90 GT lights on. It was a straight swap. If you look at my '84 LX link in my signature you'll see that i got later model lenses to work on my car. the '79-'82 lenses i believe are the type that are different than the '83+ lenses. LX's and GT's from '83 to '84 i believe had the black stripe in the middle and '85 or maybe '86 and up are the solid LX type on both GT's and LX's. In '87 the GT's got the louvred style lights until '93 and the LX's got the plain blocky type from '87 to '93. The cobras got the '83, '84 (not sure about '85 or '86) style tail lights that look ALOT like the cobras except without the 2 extra mini stripes to the top and bottom of the large black stripe accross the lenses.
basically anything from '83+ should be direct swaps because all of the tail lenses are in the same place.