[nxtruck]

The oil valley drain holes were tapped and plugged to keep the oil drainback at the rear of the block. On a street/strip engine, I wouldn't plug these holes.
As far as the steam holes being plugged, like BigJim said, it is done a lot on drag motors, but I wouldn't plug them on a street/strip deal. Factory 400 blocks are known to crack from the steam holes to the head bolt holes, which, once again, isn't a big deal. In my opinion, if you're using a factory 400 block, then you should run both head and main studs instead of bolts. This will, among other things, keep the small deck cracks from the steam holes from becoming a bigger issue. And, by all means, drill the steam holes in the heads if they don't already have them.
Just my opinions...