Where exactly is the oil leaking from? from of motor, back of motor, underneath the dizzy, under the VTEC solenoid, etc etc. Did you remove any sensors or anything when you had the block rebuilt? Are we talking about a couple of drips? or is it a huge puddle?

I had one of those "moments" that Schmoo was talking about too. When I was putting reinstalling my oil cooler on my GSR after the motor got rebuilt the large O-ring on the back of the oil cooler fell off while I was reinstalling it. This cause me to leak a lot of oil. It looked like the oilpan was leaking but after looking closer it was actually up a little higher.

The oil cooler o-ring is just one of the possibilities. It could also be leaking out of the stock oil pressure sender hole, the oilpan, the VTEC solenoid or dizzy, or maybe it is leaking from between the block and head. Just look at the easy things first and work your way to the parts that would be hard to check.