Welding the crack will require a full teardown to the bare block, and the services of a very good welder experienced in cast iron welding. It's necessary to pre-heat the block, weld about 1/2" at a time, and peen the weld to stress-relieve it as it cools, then weld another 1/2", and do the peening again, etc. The metal will crack right beside the weld as it cools if this isn't done.
I'd suggest grooving the crack out just a bit with a die grinder, plus grinding the metal bare of paint for about 1/2" on each side of the crack. Get some Devcon Epoxy Steel, which is much harder and more durable than JB weld. Mix it according to directions, and apply it about 1/4" thick over the crack, tapering to nothing at the edge of the ground-smooth area. Lei it cure overnight, then dress the patch with the die grinder and repaint.
Jerry
