How do you get out the old oil. Motor is a 454/ 310hp

There's two plugs on the side, one for filling and one for draining. Depending on what kind of room you have, you can drain it without removal or simply remove, drain and re-fill at the workbench.

I only see the plug to check the level. Where exactly is the drain plug?

Some had 2 plugs some had 1, I have both on my twins. Pull them off, tip them over and drain and flush, then replace with gear oil. You'll be suprised at the crap that comes out.
BTW take your belts to napa and they can match them up. I just got all 5 preium belts for mine for 50 bucks.
And you may as well replace the sea pump impeller while its off. I would do this yearly. But then again I'm anal and dont break down on the water.

Dan

I take my off and tip them over too. Why do you use gear lube instead of engine oil?

I pull the fuel pump off and use a syringe to suck oil out.If I am doing the impellar I pull the whole unit.Use merc gear oil.If the old oil smells like gas the pump needs replacing.