If the verb in the first part is in the past tense then the second part should have 'would' and not 'will'.
He said he would come. (Not will because the verb 'say' is in the past tense)
If he didnt want to come then why did he say that he would (come) ? (here even the verb say is not in the past tense, however, there is 'did' which makes the verb from the past tense)
So your second sentence (I hoped you wouldn't mind when I told you the truth) is correct and the first one is grammatically incorrect.
more examples -
If i get the time, i will stop by at your place.
If i had got the time, i would have stopped by at your place.
If you had asked me nicely, i would have come with you.
If you ask me nicely, i will / would come with you.
See this one
What will you do with so much money ? - (You put 'will' here because you know that person to whom you are asking has got that kind of money.
would is often used as past tense of will (that sounds a bit ironic as in past tense of future)
Suppose you are asking a person about a future possibility then you frame your question using 'would'
If you win one million dollars worth of a lottery, then what would you do with so much money? (here 'will' is not possible because the he hasn't yet got that money)
Would is used in many different ways which makes it an altogether a seperate lesson to be learnt.
But for now i hope this answer is sufficient.