"Would have been" is normally counterfactual, describing something that didn't happen, for an explicit or implicit reason.
"Was" is past tense
- It was nice to see you yesterday, I'm glad I went
- It would have been nice to see you yesterday, I'm sorry I didn't go
It's not quite clear what your example sentence is supposed to mean, but "if the fire of love had started equally" and "would have found" implies it is counterfactual: so you need "would have been".
I'm not sure what "fun of the heart attachment" is intended to mean, perhaps "the joy of the romantic attachment" or "the joy of the affair of the heart":
It would have been a joyous affair of the heart if the fire of love had started equally
But "neither you nor I would have found repose" seems a contradiction: unequal love doesn't normally make calm!