I think a hose reducer is available for stock outlets. Or you could make an alum sleeve with 1 3/4 OD , slide it over the stock outlet and weld it on. I changed the outlets on my radiator to match those on water outlets.
Get the aluminum ones from most any hotrod parts supplier. You can paint them or powder coat them if you don't like the aluminum look. I'm sure Speedway has them.
http://www.speedwaymotors.com/1949-53-Flathead-Stock-Angle-Water-Neck,7313.html You need to look a little harder.
Those are 1-1/4 and not 1-3/4 Tommy. Probably why he said it was a no go. I'd just about bet that Flathead Jacks are 1-1/4 also http://flatheadjack.com/77.html The Hot Rod Company also sells 1=1/4 outlet housings http://www.thehotrodcompany.com/shopnow/show_dept.asp?dept_id=77&first=-1
I made my own.....used a section of steel pipe for the neck....cut off the original cast iron outlet, enlarged the opening .............preheated with a torch and welded with stick welder and cast iron rod.....put them into a bucket of sand to cool slowly. Worked like a charm. Ray
I know, an old thread, but here's a link to aluminum ones - he also has cast iron, it looks like: http://www.honestcharley.com/hot-rod-parts/water-neck-aluminum-49-53.html R