You can actually install High Sierra on an SSD ''without'' automatically converting to APFS, using, of course, HFS+ instead.
1) Download the MacOS High Sierra installer from the App Store as usual, making sure it’s within the /Applications/ directory. You will need the full installer to have the Contents/Resources/ options available to you.
2) Open the Terminal application, found in /Applications/Utilities/ (or from the Utilities screen menu options if booted from a USB boot installer)
3) Enter the following command syntax at the command line prompt:
/Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/startosinstall --converttoapfs NO
4) Hit return key to start the macOS High Sierra install process with the –converttoapfs NO directive, thereby skipping the APFS conversion of existing file system