Trouble With A Barn Find

Greetings!
My name is Andy and I live in Virginia, USA. I recently acquired a 1973 Triumph Bonneville T140V, 750. This was a true barn find, as the bike was recovered from a shed and had not been started in something like 40 years. I was gratified when I gently pressed down on the kickstarter and the engine rotated without much difficulty through the full range of kickstarter travel. (I have not yet attempted to actually start it). However, somewhere along the way, I have managed to mess that up. I have changed out all of the fluids and filters, cleaned and rebuilt the carbs and basically put as many things right as I can without cracking open the gearbox or chaincase, which didn't seem necessary. However, now, when I apply pressure to the kickstarter, it moves down easily to the point where it should begin to turn the engine and it hits (what feels like) a solid mechanical stop with no further movement possible. When I try to rotate the engine by hand using a ratchet on the crank (on the left side), same result, hard stop. I installed a new clutch cable, but the clutch does not seem to be functioning, either. When I pull the clutch lever in and put the gear selector in first, the rear wheel will not spin. But when I put the gear selector in neutral, which is not hard to do, the rear wheel rotates freely. It feels like the transmission is engaged and won't release, and using the clutch doesn't help. This seems like a simple "user error" type of thing. I haven't done anything to the engine, valves or transmission, and I know the system rotated just fine when I began cleaning things up. Any ideas? Thanks for taking the time to consider my issue.