Guest,
There were at least three different designs of the elevated center-line platform for a 20-mm gun on FLETCHER's (another similar platform used on BENSON-GLEAVES class units, plus designs unique to other destroyer classes). BosNY and NYNY had two slightly different designs for Round-Bridge FLETCHER's. A third design was made for Square-Bridge FLETCHER's. The BosNY design had a crew access and ammo passage through the bottom of the platform with a ladder under the platform. Meanwhile, NYNY had an access on the portside of the platform with a ladder run up the face of the bridge and smaller ammo passing shuttles through the bottom. The BosNY design had an issue with the access being just outside of the working circle for the 20-mm gun. I don't know if there was a hatch cover over the access or how they managed not having a crewman fall through the hole in the deck. So I have wondered if crews altered the access in the forward areas? In any case, it appears that the NYNY design was used on FLETCHER's built at other yards.
As far as dimensions of the NYNY-style platform, it appears to be the same or close to the BosNY-style platform. The most obvious structural difference, is that the NYNY-style platform has more "rounded" edges forward, while the BosNY-style has a more cutoff "square" appearance.

Getting good closeup images of the platform, with overhead views next to impossible, are rare. Here is an image of the platform installed on a sister, USS HUTCHINS, built by BosNY that had the same platform installed.

Here is an image of USS SIGSBEE with the NYNY-style platform showing the access opening in the portside bulwark and ladder location.

As for your structural questions about damage done, I refer you to the attached link to the BGP for USS SIGSBEE (DD-502). She was built at Federal Ship Bldg. and this BGP shows her in a different configuration to CHEVALIER when lost, but the below decks layout in the damaged area that didn't get separated from the rest of the ship is going to be the same. Particularly as far as location of oil tanks and such. Hopefully, these plans will provide you with the detail you need. I would need to guess what all you needed and try to post images. By studying the plans yourselves, you can assess want you need. That part of the ship would be a very tangled mess of structural metal, electrical, piping, etc., from the explosion and there are no photos and the Battle Damage Report is somewhat of a best guess. So, there really isn't an absolute appearance in trying to model the damage.
...
https://www.hnsa.org/wp-content/uploads ... /dd502.pdf ....
And
YES ... when the torpedo hit and exploded the 5-in ammo magazines, the degree of destruction is great.