Owned a 77 and 79 T/A, also built a 75 and 73 for friends. as other have said, the timing chain is the weak point of this era 400. Being a 79 and factory 4 speed, it should have the 400, and appears to in the pics. Pontiac was basically out of 400's by the 79 model year, and the 4 speed cars had priority. (Don't see the oil filler on top of the motor as you see on the Olds 403. that being said, if the car has headers, plan to get factory manifolds, as the headers are so close to the starter the heat kills starter solenoids, (trust me on that one). The door gaps on a T-top car concern me, as others said, the cars were not that rigid even without T-tops (found that out doing the auto-crossing/road racking thing), so I would take a very close look at the floorboards/floorpan. It appears the car is resting on the suspension,(fronts on ramps and jackstands appear to be under the rearend housing). and most concrete pads are relatively level, so I tend to not think that contributes to the doorgap appearance. Cannot tell from the window sticker if this was a "stripe delete" car, I would be looking very closely for evidence the car has been painted. Applying the bird is not extremely difficult (I made a career of it), but it would be tough if you only put them on once a decade. If the window sticker does not specify "stripe delete," I would be looking very closely at the windshield/rear window trim for signs it has been removed, as well as the screws that attach the frames around the headlights. A paint job might also be why the grilles are missing. Check in the trunk for patchwork/bondo in the lower quarters, that and the rear window corners were the water traps on these cars. Good luck, I'm on the fence about looking for one of these too.
