Sounds like your rear most cam bearing was installed incorrectly, if this is the first time you have cranked it since the rebuild.
Could be a set of clogged push rods it you reused the old ones without thoroughly cleaning them (should be able to see down the length of the tube).
If it has been running for a while since first crank up it could also be carbon from the oil galleries that was loosened by boiling or baking the block that was not flushed out by thoroughly cleaning the block prior to assembly of the motor. Therby clogging up your hydraulic lifters to the point that they will not pass oil. Though clogging all sixteen lifters is highly unlikely.