The differences in a 1998 is a Gen I Smallblock.  It is 'basically' the same block as what was used in 1987.  Some mild changes took place but it is still the same.  The 1999 Truck (not Van) engine is a LS based Gen III engine and is shares no architecture with the Gen I small block.
The positive is it will drop right in.  Gen the engine, wiring, ecm, manifolds, partial exhaust as much as you can from the donor.