It's "safe", electrically speaking. I would think that it should work OK if you configure the arduino environment for a "NG or older with ATmega8" board rather than a "deicimila"; a lot of the configuration that happens there is more rom monitor and binary cpu compatibility, rather than "board" compatibility. For instance, pre-diecimila ATmega168 cpus work fine in the "Freeduino" Diecimila clones.