I had the same problem on my Prostar 190 when I first bought it. I removed manifolds and risers. Used a small oxy-acetalene rig and heated them up and unscrewed them because they will strip the centers out if stuck or it breaks off a peace of the plug. You can heat the plugs first then heat around them. Also good time to take risers off and clean and inspect both riser and manifolds. Clean sand,wire brush and repaint exterior and install these drains with plastic hose on them long enough to stick through drain plug hole so no rusty water drains into bilge. Also replace motor drains with easy drains and hose too. http://www.skidim.com/prodinfo.asp?number=MCM000
Then put back together with new gaskets and new bolts.