You can use the heads and just leave the AVCS solenoids disconnected, they will just act like normal cams.
As mentioned you can't use a MY98 manifold with the sti heads, but you can put MY98 wrx heads onto the 2.5 short block. If you have a MY98, this will be the easiest option.
To use AVCS your going to have to change to a motec, autronic SM4 or linkPlus G3. All would have to be wire in for a MY98. The SMC does not do variable valve control.
You could possibly change to a newer subaru
ecu, but this is ALOT of work, requiring the newer dash and vehicle loom.