I'd imagine that the 350 and 403 would be easier to find, they're newer, and probably less in-demand than the 455. The 350 and 403 both have the same deck height as the 307, so they're technically closer to a drop-in swap. The 455 has a taller deck height, so the engine is taller and a little wider, and it uses different components such as intake manifold, brackets, and headers if you go that way. If you were able to find a fully-dressed 455, that would almost be a direct drop-in, except you'd probably have to do something about reconnecting the exhaust. The 350 and 403, I'm PRETTY sure that everything would be interchangeable, although if you got a motor that didn't have an intake, I don't think you'd want to swap over your 307 intake. But accessories should all bolt right up. Of course, if you're going in the direction of a swap, you'd definitely want to get the carb and distributor, as you're not going to want to move the CCC stuff over to a motor of a different displacement. I think it's technically possible, but not advisable.
Another consideration is transmission. I'm not sure what you have in there, but I'm guessing it's a 200-4R. Totally pulling this out of the air now, but those are fairly light-duty transmissions from the factory, but I believe they have fairly decent aftermarket support, as they also came in the Grand National. If this is it, I BELIEVE it could be beefed up. If you decide to change to something else, that could be opening a can of worms, with driveshaft length and crossmember pad location.
I'm pretty sure you have an 8.5" rear in there, which if that's true, it should hold up to the power, but gearing might be lackluster. I really don't know what gears it would have, but it could be something from 2.41 to 2.93. (Could also be something totally different.) Depending on what you have, a gear swap could actually be a pretty good supporting mod, if you were planning to keep the car and eventually upgrade the motor. If you had something like 2.41s, this is something that could be done sooner rather than later, and would probably make it more fun to drive in the short term. If you already have 2.93s, those actually aren't awful, and I think would work ok.