The Classic and Plus analogs are not interchangeable.

Analog boards that are "common designs" include:

128K/512K/Plus

SE and SE/30

Classic/Classic II (Revision A)

Classic/Classic II (Revision B)

(see my FAQ in the Compact Macs forum for more information about Revision A and Revision B)

If you want a new analog board, look for a pull from a Platinum Plus as they have more reliable power supplies than their beige predecessors.