Hi!

Overhang depends on both the length and the construction of the rolling stock (position of pivot points etc.).

There is a table in the NEM you can use as a rule pf thumb.

See NEM 103, NEM 112, and others in the low 1xx range.

As I read it you need about 13 mm extra clearance on each side when using 303 mm coaches with 400 mm curve radius. If you stick to "new longer length" shortened coaches, 10 mm extra should be enough.

At 57 mm track spacing and using 360 mm radius for the inner curve, you do not have those 10 mm. So expect problems even with "new longer length".

The 77 mm used by Märklin may have a few mm in reserve, but with scale coaches you probably cannot go much lower than that.

Flex track is not going to work at that tight of radius.

You are much better off using standard R2/R3 curves with the 64.6mm spacing for your parallel curves.

You can then either keep the 64.6 for the straight sections and use the slim turnouts with the 30mm spacer to get the wider spacing, or you can transition from 57mm to 64.6mm at the start of your curves.