Your bike will behave best as Triumph made it; I have no experience of T140 isn't that a 750?, in which case it will be pointless.
Just larger valves will do NOTHING for your performance.
Don't worry about the breather tube,if ts ben done properly there should be 3 x 1/16 holes behind the engine sprocket at about 5o/c.
Clutch cable adjustment need to be done correctly, see on a Triumph site 'cos I can't be asred to write it out right now.
Your crappy gear change will 99% be the pressure plate not lifting properly causing drag; when the engine is running with cover off and lever held in, the pressure plate should stop spinning. Adjust with the spring nuts to get an even lift.
Unless I knew the person who built the engine I'd never trust the rebuild; cruel but learnt from experience.
Carbs; I prefer monoblocs but it don't really matter. You can get new monoblocs from Burlen,Salisbury,don't expect then to hold your hand.
Took me 20mins to figure out what pcc was.
Good luck, a cracking bike when properly set up.