Fresh 340 rebuild issue

Hmmmmm..... Those rings should break in easily, but running 1.5 hours un-loaded is not a good way for ring seating. Having some real torque load on the engine from driving is normally needed. I would not be running it on blocks myself, for this very reason. Get the car on the road ASAP....

In my one experience with a bad ring installation, the oil smoke started up right away, and never changed. (It was a doozy... used a quart in 20 miles!) So I am inclined to think it is more likely an intake leak that has developed as suggested. Since this car is on blocks, you will have plenty of vacuum in the intake manifold even at higher RPM's with no engine loading, and that will make such an oil leak show up.