First of all, 「好{す}き」 is an adjective and not a verb, meaning there is no such thing as present or past "tense" for the word. If you knew this, I would apologize, but I had to mention it after reading your comment like "making the 'suki' past tense".
Thus, your question is actually about the tense of 「だった」 instead.
「ずっと好きだった。」
can mean both:
1) "I have always liked you."
2) "I had always liked you."
This "fact" always surprises English-speaking Japanese-learners because to them, those two English sentences mean very different things. 1) says "I still like you." and 2) can mean "I no longer like you."
Japanese is a highly contextual language. What the sentence 「ずっと好きだった」 means in terms of tenses as defined in English depends entirely on the context. In this particular case, it depends on whether the temporal reference point (understood by both speaker and listener) is at the present moment or a time in the past.