Can a 1998 Chevy 350 head fit 1984 block?

No, a 1998 Chevy 350 head will almost certainly not fit a 1984 Chevy 350 block. While both are small-block Chevys, there were significant design changes to the cylinder heads over those years. The bolt patterns, valve angles, and even the intake manifold bolt patterns are likely different. Trying to force a fit would be extremely difficult and likely cause damage.