Q&A Question referenced for others
HERE with current as is chest plate only fitting Neo Crusader (NC) Era without modification.

Looking to add two oblique plates that fill in the negative space left by the curvature of the original plate. Obviously my Paint edit is a pretty rough idea, but these oblique plates would be similar to the shape of the cheek indents on a modern helmet; they would follow that triangle with a curved hypotenuse, and the plates as a whole would be fitted to curve around the body
The most common crusader plate is seen below in the reference image.



Fitz from Krayt Clan (above ^ ^ )
Ru'Der Marekar's WIP has a similar style (also seen below).
bh1138's WIP also has similar styling.
Highly recommend this WIP for step-by-step progress Ruug'la Carud's current WIP
Alternatively there are many styles of Crusader Era chest plates. See image below to the left behind the female Jedi in brown armor.

The reference below is similar to my approved crusader kit and my kit is derived from ramikadyc style armor.


My personal
Late Crusader WIP.

Another reference image:
Zophar Ptay very detailed late crusader WIPVeeds is current in progress as well for their
Crusader WIP.


Another common style is the use of a hose protruding from the chest plate (as seen below):

Bernie's Late Crusader WIP is really good for how to make the above armor. Referenced images below:



My thoughts on your plates to make them fit the era would be to make the current plate above into two separate pieces. The top section be the chest plates and the bottom section be the ab plate. The ab plate will need to be trimmed and modified to not appear like Neo Crusader plates.
Alternatively you could go more the route of the crusaders with hoses attached to their chest plates and modify your chest plate to look more like the image below for reference with ribbed flightsuit and hoses to the chest plate instead of helmet from backplate.

Your helmet reference of
Shae Vizla below works perfectly for this era of crusaders.

My last piece of advice would be to start over for the chest build. Trying to modify your current chest plate will cost you more money and time in the modification process.