The core thickness is not the issue in putting the older (89-91) brass tank style into a newer (94-95) SC. The way the radiator bolts to the top radiator support is the issue. There are two different mounting styles. The 92-93 should be a direct bolt in.
The early style has one bolt on each side that bolts to the upper radiator support through a rubber grommet. The late model style has a bracket on each end that bolts to the upper radiator support but attaches to the radiator via a rubber doughnut on the bracket that slips over a pin on each side of the radiator. (This info is for stock radiators only, I have seen aftermarket radiators that have different mounting systems!)
The problem is with the spacing of the nutserts in the upper radiator support. On the newer cars these holes are further apart. So when you put an early radiator in a late model you can only get one bolt hole (usually the passenger side) to line up with the radiator. The driver's side radiator mounting ear is then about ½-1" short of lining up with the hole. Extending the ear with some metal or making a new nutsert hole may solve this. You might also be able to slot the holes and split the difference between the two sides.
The other thing that is different which way the overflow tube points. It is on the passenger side for early cars, and driver’s side for late. I think this actually started in 93.
I know I bought a 94 with an older brass radiator and they just put the bolt on the passenger side and left the driver's side without a bolt. Not my first choice for a solution!
If you have actually mounted the early brass radiator in your newer (94-95) SC please post how you dealt with this issue.