par96
Cory is correct on his nomenclature. You will likely have to drill the hole 2-1/2" deep if you need 2" of thread engagement, to allow for the imperfect threads of the taper tap ( I wouldnt really suggest using a bottom tap in 4140 unless in the annealed state.) and to allow chip clearance.
Couple of other things to consider, I would look at an Allen head bolt, I cannot recall finding slotted head fasteners that large, and you have to be very careful about any loads this filler might see, countersunk bolts aren't really structural in nature, they will fasten a filler piece on just fine, but any loads subjected to the filler could cause failure.