Should I show the income from a sale of ancestral farm land as regular income or long term capital gain. the sale was at a lower than fair market value.

Enter it in the investment section as land. Your basis is the FMV on the date of death. You may not have much of a capital gain. You may have a loss if you had expenses of sale to add to the basis.

Thanks.  It is going to be tricky to get FMV because the donor deeded the land about 30 years ago.

Did she deed it while she was still alive? Then it is not an inheritance, it is a gift. You would use the original owner's purchase price when they bought it.