The front end should be a cascode diff amp. Two 5687's are a fine choice. They have low enough plate z to allow an Ec2 of ~70V, and gm for enough gain. Figure B+ of ~300, and plate loads of 25-30k.

Special voltage ref for Ec2 to the common cathodes instead of the traditional ground. Building a voltage reference is no biggie. Just assume it is as I say for the time being....
regards,
Douglas