I absolutely love the Quiet Line models! They are about the nicest looking garden tractor IH ever made, and really have no more problems than some of the newer 82 series.
Being one of the most produced model tractors, you often will get a number of replies. I happen to have a 1450, that I have gone through and upgraded. I'm currently going through a few more upgrades, and cleaning up a few parts.
Your 1200 looks nice and fresh! Congrats!
The engine cradle modification, for those who actually done it, is a very simple procedure. I don't know if there is on official procedure here at OCC that discusses it, but you should use a bolt on brace for the rear of the engine cradle. Sometimes the oil pan does not serve as a secure jig, and those two ends of the engine cradle can turn in from the heat of the welding procedure. You will want to decide if you want factory replacement engine mounts, or some have had good luck with car suspension parts, and solid steel mounts so I read. I went with the factory replacements after reading Myron Bound's comments and opinion of them.
You ought to keep an eye out for a cast iron oil pan, and maybe add a Kirk Engines muffler brace at some point. I can't say much for the clutch aspect because I never had a all-gear Quiet Line to share in the experience your having.