How do I build a 90 mm eye-to-eye shock for a Team Associated RC10 B6.1D?

A 90 mm eye-to-eye shock is not a normal build target for the RC10 B6.1D. The setup information for this car lists short or long eyelets, but does not give a 90 mm shock assembly specification.

For the B6.1D kit setup, the front eyelets are long and the rear eyelets are short. Build the shocks with the correct front or rear parts and don’t add long rod ends just to reach 90 mm, since that changes suspension geometry and available travel.

Do you mean 90 mm measured between the centers of the mounting holes with the shock fully extended, and is this for the front or rear? If so, please check that measurement and tell me what shock parts or replacement shocks you’re using.