68 Barracuda Fastback -best fitting roll bar?

A good compromise is having one built to specs you provide. For example, I followed the instructions provided by Alston Chassisworks and got a killer cage kit for my 66 Dart. It fits like a glove. The crucial components are the main hoop and a-pillar bars. Everything else is either straight or trim to fit.

https://www.cachassisworks.com/Attachments/WorkSheets/CustomCageInstructions&Worksheet.pdf