Those 496s live alot longer than they should , considering what they are. Personally ,i think the improved head to block sealing is one of the keys. Head gasket failure does real bad things to those 502s

I have 860 hrs on 2003 496 mags and they run perfect, only thing I do is change oil every 30 hrs and leave the motors stalk !!!!!

Don't be worried about 400hrs if the motors are stock and the hour read outs are normal operation (lots of idle time, little above 5000rpm). These motors run 1500hrs+ in cruisers. The guardian system lets you know if anything is going wrong before it can do real damage to the motor. In stock form the 496s were/are some of the best.

Like mentioned before, if the run time history is normal and a compression/leakdown test is passed, you have alot of trouble free boating ahead. Bought mine with 387hrs. Broke 500 last summer. 2 years on the Whipple Stage 2 PCM tune. Runs like a top. Keep up with maintenance and they will last.