Tax loss harvest to reset basis?

In my brokerage account I have a stock with 8k of losses ( been holding it for years before switching to boglehead strategies). And another stock (index fund) with 10k of gains. Is it advantageous to sell the loss and sell 8k of the index fund to reset my cost basis of the index fund? Or sell and tax loss harvest during my tax return for the next 3 years?

Thanks for the input