Hard to tell from just the photos but a few observations raise a few questions.

Chassis appears to share the same width as the ECO and not as wide as the 7. Combine this with the balance that is achieved using a 3S pack vs 4S.

The nitro 7 moves the center diff slightly to the right (compared to the 6 platform), moving the nitro motor over as well. When converting using a 7 nitro chassis, if you maintain the battery tray distance as that on the ECO (in relation to the center diff position), you are also moving the battery tray over slightly.

The 7's servo saver arm is angled to match the angle of the steering servo. Straightening the servo (like the ECO) now creates a slight angle variation in the servo horn to the servo saver arm. Granted, the EPA on your radio will compensate for this.

When converting my 7, I did my best to maintain the weight and balance of the nitro car and maintain use of the popular 4S/1900 battery motor combo. This meant leaving the center diff where it was and moving the battery tray over with it. I also felt that maintaining the steering servo linkage geometry was important in addition to gaining even a slight weight bias from the servo positioned slightly more to the right.