is there a difference with the heads on a 98 classic and an older classic ?
are they interchangeable ?

 

Yes, MY93-96 phase 1 heads do interchange with MY97/98 phase 1.5 ones and, therefore, the inlet manifolds.

The MY97/98 ones are more freer flowing than the MY93-96 ones, as the early ones have limiting hydraulic tappets. The phase 1 WRX ones are the runt of the litter so to speak, and give their best at around the 380bhp mark. The phase 1.5 under bucket shim STi ones, by contrast, are capable of flowing 500bhp+. Therefore, these would be the heads of choice, if wanting to upgrade from early phase 1 ones.