The 198 slant six was made from 1970 until 1974.
It was basically a destroked 225 with different rods that replaced the 170.
They weren't all that common, but they are around here and there still.
The 5th letter in the V.I.N. would be a ''B'' for 198 and a ''C'' for 225, and came only in a-bodies.
A lot of people that build slant sixes like to use the rods in their 225 build ups, so if you run across a set, don't throw them away!