Look in the owners manual, under "Winterization", and it will show the locations.
The following is based upon a 5.0, closed cooling, with Bravo 3. The 350 should be very similar.
Facing the front of the engine, if I remember correctly, 2 plugs are on the left, at the water pump assembly, one in front, and one on the side, and is hard to see. I used a flashlight and mirror.
Also, one plug is on the right, at the base of the fuel filter/cooler. Also, one plug, is at the top of the heat exchanger, which is removed to vent for draining, for a total of 4 plugs.
Hope this helps...
I noticed you didn't say you have closed cooling, but some of the replies are talking about closed cooling.
I don't believe the closed cooling systems have that many plugs. If you just mean the drain plugs, be sure that's what you're looking for so as not to get them confused.
FWIW, they are NOT always very easy to get too. To get 2 of mine, I have to remove the risers to be able to lay on top of the motor and reach far enough down to get them. Luckily SM risers are a breeze to remove. Previously I'd end up head first in the bilge with a fear of getting stuck! :grin: