One thing that won't be quite right is the RIAA eq applied by whatever preamp your turntable is connected to -
1. Record the 78 at 45 or 33 1/3 2. Apply the Inverse RIAA EQ (to make it like a "flat" recording) 3. Change the speed to 78rpm 4. Apply appropriate EQ for what the 78 was recorded with
So, playing a 45 at 33rpm (and vice versa, presumably) messes up RIAA playback eq? I did not know that.