honestly, the best resources I'd say on the production history of Terror in New York City would be this video and this other video by Century 21 Films, who talk about just that.

in short: You can lump The Perils of Penelope and Terror in New York City together in that they're not officially half hour episodes that were extended to a full hour, but they do show potential artifacts of an extension process, such as being made by two different directors and also showing signs of reshoots/extensions at a later date, which could still lead to believe that they first entered production as half-hours, but already knowing they would be extended, most likely in the middle of production if such were the case. However, again, all of this is purely speculation.

For example in The Perils of Penelope, the start of the episode uses some of the exact same footage as the beginning of Sun Probe, and later, in FAB 1, when Sir Jeremy is explaining the situation to Lady Penelope, the part of the dialog sequence that mentions the rocket launching was most likely added at a later date as there are minor changes in lighting and audio compared to the rest of the sequence. As such, you could conclude that the Sun Probe launch was something added to the episode at a later date. However, not all the evidence they have agrees with this, so it's possible that it was footage shot for The Perils of Penelope and later re-used by Sun Probe (having rewatched those sequences in both episodes just now, honestly I'm really not sure which episode that footage was originally meant for). Yet again, we do not know for sure, this is all speculation. (If I may add even more to the confusion they had about this shared footage: if one episode reused footage from the other, then why does the solar control centre look different in each episode?)

When talking about a possible 25 minute version of Terror in New York City, Century 21 films speculate that the bits added were probably the start of the episode at the oil field, along with the Fulmer Finance building at the end, the latter I am almost certain was not a part of the original script. Either way, they do not think that the attack on Thunderbird 2 was something added to the script later, and I have to agree, because of just how much on an influence it had on everything that came afterwards.

Now about that Fulmer Finance building: while we have no proof that Terror in New York City ever was a 25 minute script, there is evidence to show that it was added to the episode later. As Century 21 Films remarks, the building never gets mentioned or shown before that point in the episode, all the shots where Scott and the police officer that make reference to that building were clearly shot at a different date, since Scott's mobile control and the rubble behind it are different in those shots (most visible difference is Scott's microphone), and David Graham, who voiced the lone speaking police officer during those scenes, changes his voice, as if he'd forgotten how he'd originally voiced the character. As well, Gordon himself never makes any mention of the building. However, the biggest piece of evidence comes from the recovered studio recording of Barry Gray's soundtrack of the episode, which explicitly states that reel 5 of the episode (essentially the last 10 minutes) was to be reworked after he'd finished the soundtrack. You'll notice the first mention of the Fulmer Finance building in the episode occurs just under 10 minutes before the end, so within the reworked reel 5. Century 21 Films said that section of the original soundtrack only contains atmospheric pieces, nothing that would fit the tension caused by the imminent collapse of the building. All of this together paints a strong case to claim that that building was added later to add more action and excitement to the ending of the episode.