It sounds like you may be asking too much of your B&M 250 to make 8# of boost for any stretch of time with no chiller. It's got to be making some serious heat turning that hard. It does seem odd #8 got lean, but it may have just broke 1st. Look at other plugs with magnifying glass to see if any material present. If you plan on running it there going forward I would either put a chiller on, or get a bigger blower and turn it slower to make the same boost.

What heads and cam are you running?

I am starting to think that it possibly did run lean, but the weird thing is everything else looks really good, and why did No. 8 burn a hole. That is what's puzzling me. I will take a closer look at the plugs. I am running the stock mercruiser / Carter fuel pump. It is supposed to flow somewhere around 115 gph. I was told that it would be enough? One engine it stays at 9 psi and on the other engine, which melted down it stays at 6 psi. On that run I am pretty sure that there was not a drop in fuel pressure. I bought new Monster gauges and senders. They are mounted on the top of my dash right in my line of sight, and I was constantly checking the gauges because everything was so fresh.
My heads are Dart iron eagles, and my cam is a special grind Isky that the engine builder used before and had good results with on a similar set up.
Greg

JJones, when you leaned out your engines did it burn up just one piston or did you find damage on other pistons as well?

Just went through the same thing on similar setup, 502, intercooled, B&M 250. Burned the end pistons.
You ran lean, especially at 8psi, no intercooler. That's a lot on a marine engine.
Lower your boost, upgrade your fuel pump, filtering capacity, remove anti-siphon valve. BTW, It's not always a loss of pressure, it's the flow volume required to fill the carb bowls.
Gary

I would bet that your fuel pump does not flow nearly enough. Remember, pressure is not the same as volume. What size pick up do you have in the fuel tank? Does it still have the stock check valve? A fuel system is only as good as it's weakest link.

My holley 750 carbs are set up with 78 pri and 86 sec with 4.5 power valve in primary. This is about identical to the older merc 600sc. I am running the 155193 pulley which gives me 6 lbs of boost. 454 30over with 741 cam. Dyno at 625hp. The 155194 pulley gives me 575hp and 5 lbs boost.

Doesn't look like you are giving it enough fuel.