There should be three galleries running above the cam shaft from front to rear . The top center
oil gallery isn't the one you block to eliminate the lifters. I now have a 427 sideoiler. I've had 428s and 406s in the past. They all had those three galleries. The lifter galleries may not even be drilled as most 427 sideoiler were solid lifters. There were some T-birds (the big ones) that had hydraulic liftered 427s. I never saw one in person just read about them. Seems like they were rated at 350 hp. Had to be more than the 390 at 335hp.
With of the increase of usage of hydraulic roller lifters Id bet that some of these old blocks are having those two lower galleries to either side of center gallery being drilled from end to end by machinist today.