If agricultural property relief APR (or as an alternative, business property relief BPR) may apply to the gift, you should claim it when you send in the inheritance tax forms prior to the probate application. It will not hold up probate, so go ahead.
HMRC may raise queries about the APR/BPR claim in due course, so you can't distribute the estate until any queries have been addressed and IHT clearance has been issued. However, in the meantime you will have been able to obtain probate and deal with estate assets and liabilities of the estate.
IF APR or BPR applies to the gift at 100%, you will not have to pay any IHT and the gift will not be taken into account in calculating the IHT (if any) payable on your father's estate.
If HMRC does not allow the claim for APR (or BPR) you (as recipient of the gift) would only be liable for tax if the gift took your father's total gifts to more than £325,000 in the last 7 years before his death. The rate of tax on the first £325,000 of gifts is 0%, so if the gift of land to you was the only significant gift he made in the last 7 years of his life, the rate of tax on that gift would be 0% and you will not be liable for any tax. However, the gift will have used up a small amount of his nil rate band and so will slightly increase the amount of tax payable on his estate.
Taper relief is only relevant if the gift to you is taxable at 40% (ie not at 0%) as this relief operates to reduce the rate of tax, not the value of the gift.