Unfortunately, version 9.0 is the earliest NuGet package available. Version 8 is very much out of support/active development now.
You can try referencing all the .dlls in the
\Lib folder from the .NET API project that React is interacting with.
You are correct, both projects will need to be using the same versions of the Kentico library binaries (including the same hotfix version).
Another option is to create a minimal .NET API in the CMS application project using
I would recommend upgrading to at least version 12 of Kentico, which is the last version to support the Portal Engine development model you are using in your Kentico 8 site. Or, consider a rebuild in Kentico Xperience 13.