assumes that the lump sum he is giving you is marital assets that you do not have to pay taxes on).
how would they Not be marital assets?
If he gives you a lump sum at the divorce that comes from, say , his half of the house equity or from non-retirement savings accounts - that's just a division of assets and there are no tax consequences.
On the other hand, if the lump sum comes from him giving you his half of the 401k - you will pay income tax on that 401k money as you withdraw it. (I'm not sure but I think there's an option for a limited time around the divorce to withdraw money from the 401k without having to pay the 10% early withdrawal penalty, but you still have to pay ordinary income tax on it unless you roll it over directly to an IRA or another 401k.
Also, btw, you CAN take distributions from your 401k earlier without penalty through Section 72(t) under a rule about substantially equal payments. It requires you to take even payments for five years using one of three amortization schedules; since you're so close though you may not want to tie yourself to that withdrawal for five years.
