I own a 1996 350 vortec motor in a chevy trk.bought it new, zero miles. swapped out fuel inj.sys.for a carb.?

besides changing intake an distributer,an the water pump.this engines never been apart for any major repair.like for instance,timing chain/bearings/seals.this engine has 400,000 miles on it.the oil has been changed religously....how long will this engine last before bearings need to be changed?