On early and mid 90's factory BBC (330-415hp) Mercruisers.....What is typical regarding total hours before rebuilding/replacement/major service?

They're still running?

I asked the same question once and the answer depends on a lot of different possibilities - fresh factory 330hp were good for 500hrs to 750hrs provided they were employed in their design-load environment.

Wassup?

Have a knowledgeable engine guy do a leakdown test. This will tell you exactly what issues are inside those motors and where. I wouldn't want to exceed 400 hours on any top end, even a low-po 330. While you're in there, you can visually inspect the lifter faces and put a dial indicator on them. Valve springs cause alot of dead motors. So do leaking exhausts. 400 hours is a long time to not look at your exhaust gaskets.

I have a 454 MPI 310horse....700 hours and some change. When it lets go I will build something better...the damn thing wont give up the ghost.

Is it the original? Very nice..

I just changed the t-stat in my 1974 Checkmate 18' 165Alpha, the date stamp was April 1972....but the inline 6 will still do 50mph(GPS) without any trouble. I have preformed routine maintenance yearly and replaced the valve springs and cam about 15 years ago...

Should not have said that.

Yea I know. But if it does I'm prepaired.

610 on my 454 330 , 1988 vintege , just replaced heads with some better performance heads and the old ones still looked great , the bores in the cylinders shine like mirrors , i know the exhaust has been replaced before , and was replaced again this go around , thats the biggest cause of failure , i'v heard of 1000 + hours on the bottem ends as long as you dont beat em up too bad , i personaly keep wide open throttle runs to under 2-3 minutes at a time , mostly cruising at 3000-3500 rpms , check the oil all the time , and watch the temp guage