The coolant passages are different but they work. Ive run a set of 94 standard 3.8 heads ported stock valves to the tune of 317 plus rwhp repeatedly and reliably thus far..ONLY because I got them already ported and cheap.
Differences are in coolant passage size, deck thickness, valve stem thickness all accounting for the SC head being asturdier peice. Later SC heads have a harder deck surface as well. Overall teh SC heads are more stout.
Some standard 3.8 heads have also have a different number of intake bolts holes. Ive also been told they may flow slightly better due to thinner valve stems. The best of the 3.8 standard heads are the 96-98 I believe..
All that being said just get a set of SC heads from someone if you really need new heads