Legally you may put a 1998 engine into a 1999 car ONLY if it is an EPA certified configuration (i.e., the emission control systems of the 1998 configuration MUST be as good as or better than the 1999 configuration). If this is not so, then you may be guilty of engine switching tampering with emissions controls. The shop that does the work can be held responsible for fines of more than $30,000, and you to potentially more than $6,000. Volvo can tell you if the configuration will substitute. Believe me I KNOW.

If you chang out the complete fuel/ignitionn systems, maybe. The 99 has variable valve timing, and electronic throttle control. The 98 does not. The engines will phisically interchange but you would have too many problems with wiring, ECU's, etc. Best to find another 99.