Both "hubiera dicho" and "había dicho" would be grammatically correct. If you say "lo que yo había dicho", using the indicative mood, that'd mean literally "what I had said". If we use "lo que yo hubiera dicho", using the subjunctive mood, that'd mean "whatever I may have said". The second option with the subjunctive implies uncertainty, doubt, unknown information...
I hope this helps.
Sign in to submit your answer
Don't have an account yet? Join today
Test your Spanish to the CEFR standard