Just found this forum and thread. I had seal replacement problems on my ** 50b 1976. If you have been able to remove the circlip as described above, you might consider that as the gland needs only to be held in place against internal pressure, the circlip gland can be turned down between the circlip groove and the outer end, enough to allow the gland to be pushed into the cylinder past the circlip groove, allowing the circlip to be refitted easily from inside the cylinder, or removed again in future. In my case the circlip was too rusted in to remove as described above, so l carefully cut off the top of the cylinder just a bit further along than the circlip to remove the rod and replace the gland seals. Before welding the end back on a bevel was made on both faces to give a good deep weld notch, but not so deep as to risk any weld getting through to the inside. Tack welds were made with the rod and piston in place pushing the gland up against the circlip to get the end square and concentric. Gland, rod and piston were pushed further in to keep cool during further welding.
You're welding procedure is flawed because you didn't get 100% penetration. You want about 1/16" weld penetration. I'd guess the barrel is about a 1/4" wall. With your weld it could be less than 3/16" wall and be a failure waiting to happen. If cutting the cylinder was the only option the best way to weld it would be to bevel both sides and have a 3/32"-1/8" gap. Then do the root pass/full weld with Tig so that there is no sparks or weld spatter can go in the bore. Mig could be an alternate process since the bore needs finishing after welding. After welding, the penetration of the root pass should be machined and honed to the proper bore size. A cylinder is not much different than any other pressure vessel but operates at considerably higher pressure than most. You don't want a cylinder blowing up.