Don't. You do not need a plug in the bottom of your fuel tank.
What you need is a primary fuel filter and water separator through which the fuel will flow before reaching the engine's fuel filter (secondary filter). The most popular filter used in sailboats is made by Racor.
I'm surprised that there isn't one installed. One on line information source here: Marine Fuel Filters
Stainless steel (or most likely Monel for a fuel tank), is most corrosion resistant when it is highly polished with few or no scratches or pits. Welding will introduce pits and crannies where water (heavier than fuel) will collect and corrode. A drain plug just introduces an element that can corrode or fail in one way or another.
You can, and should, regularly check your fuel to be sure that your fuel tank is clean. Fuel sitting in your tank that has not been treated with biocide can get pretty dirty with algae growing in the tank. During your refit you should inspect and clean out your fuel tank, using something that can clean the tank sides. The amount of effort you need to expend is determined by how dirty you find the fuel at the bottom of the tank (pump the fuel out into a clear container to see how dirty and contaminated with water it is).
Another handy device for diesel engines is a vacuum gauge installed between the fuel filter and the engine to warn you if your filter is clogged and flow of fuel to the engine is restricted. With a clean tank and new fuel filters this should read little to no pressure. In our case, it warned us that the fuel filter housing on one of our engines was clogged and/or defective, restricting fuel to that engine.
Some sailboats have installed two filter housings separated by a valve so that they never have to worry about repriming their engine; they can swap out fuel filters by simply switching over to the second filter. replacing fuel filters should be a regularly scheduled maintenance item, just as changing oil and oil filters should be done on a regular schedule.