Having done my homework, and preped for this, I decided to give it a go, after all this IS the original Mercury engine and I'd really like to use it, plus trying to find a replacement period flathead unit is really going to be tough.
First things first. Preparation! Clean out the crack, identify the beginning and ending of the crack and carefully drill two holes to help stop and start the weld, 'plugging' the weld run. I'm using a dremel, an angle grinder and cold chisel to 'temper' the weld once done.. basically beat the weld with a chisel and hammer right after welding.
Cast iron will crack if too much heat is applied. But if you work in small short runs the block itself works as a big heat sink and dissipates the weldling heat quickly. To ensure decent adhesion, weld in small parallel strips.
Two of the cracks were on the flat sump matting face which made them easy to get to fix.
The other two (thought there was only 1) were on the inside block wall, but fortunately easy to get to and treat. Here the holes drilled and valleys ground out ready for stitch welding.
Finished!.. all 4 cracks flattened out and polished flat. Hope this does the job, time will tell : )