They used the 4.3 V6 in later Monte Carlos and I *think* the Grand Prixs as well. Most of these, if not all, were fuel injected. You will need to update the fuel lines, add the intank fuel pump w/ internal baffel and get the sending unit (discontinued). A buddy of mine is doing a TPI setup in his 87 MC-SS and bought a NOS sending unit (read $$$$).
Otherwise, the swap is the same as a SBC V8. The big difference is that the motor mounts go back further.