I've welded them and had them last three years or so. New ones typically last five.
I have a big flat piece of steel that, I clamp to them before welding, so they don't warp much (and as a heat sink). Use a MIG and spray down the area with a light coating of water (constantly with a pump sprayer), to slow down the heat spread/damage/warpage.
Heating the weld with a torch after welding and letting it cool slowly also relieves the heat stresses. But you almost have to have, some kind of heat absorbing product, to help control warpage.
The first couple I welded, never really broke at the weld, but next to it. The way I do it now, they may crack again, but rarely near the weld, usually someplace else.` The first one I welded, without clamping it to something, required some muscle to get the bolt holes to line up again and didn't last long.
Using the old gasket is asking for trouble. To much work, to skimp with the gasket. Though in an emergency (road repair), I've coated both sides of the old gasket with grease and then tightened it in there. The grease bakes to a crust and actually works pretty darned good. Copper grease, works even better.
Mine often crack, when I hit the cross over pipe on something. Sometimes they just crack on there own, maybe from splashing them with water, while hot.
The Renix exhaust manifolds are hard to remove, but often fairly easy to weld. The later model manifolds are easy to remove, but the cracks are often in a spot hard to impossible to weld (in the center of the collector).
