You would roll 85k into the pre tax 401k account, then convert the 20k of IRA basis to Roth tax free. Note that you are not allowed to roll any post tax IRA money into your 401k, nor do you intend to, since the 85k is all pre tax (you have not paid taxes on any of that amount yet).
Double check your 8606 forms to be sure that the 20k basis total is correct, since if you roll any of this basis into your 401k plan, it will result in a disallowed excess amount and that would have to be distributed from the 401k with allocated earnings, and this amount will be lost from your retirement accounts.
Otherwise, you are on target. The hard part you have already done in documenting your IRA basis on Form 8606, the last of which should show 20k on line 14.
Of course, if you choose to just roll 80k into the 401k and convert the 5k of taxable gains to your Roth IRA, you can do that, but since you will be taxed on the 5k, this would be a bad idea unless you are in a low bracket this year.