OK, my 1st rodeo was back in 78 when i installed a 78-400 in my 68 FB.

The 403's used in 79 TA's use the same "clamshell" mounts as the late model 400 TA's did.
403's also came with an engine mount bracket simular to the aftermarket "late engine to early body" aftermarket adapters, but with more bolt holes.
I have a set, and they look like they'd work.
May need to massage the holes some to get'm lined up good from engine mount to frame bracket.

Now, back to 78.
What YOU will have to do is get 2-6"x6" 1/8th steel plates to mount to the original frame and rear brackets in your firebird.
Once you have them located in place on the frame, lower the engine down and mark where the engine mount holes and plate to frame holes need to be drilled to bolt everything together.
Also mark around the mount to plate to trim excess if desired.
Then raise the engine up enough to remove the plates and drill