if your fitting gas you may benefit from rpi's ignition advance amplifier if your running an electronic dizzy (no points), well worth the money. also, some of the plug leads they sell then open your spark plug gap a little.
id stick with a couple of well balanced SU's. Either that or convert to efi.
webbers and the like are hard work, they fall out of tune quickly and are quite expensive.
Doesnt sound like a clutch problem, i reckon it'l be the input gear going into your gearbox rattling against the gear next to it. Putting your foot on the clutch stops it from rattling because it effectivly removes the some of the vibration being transmitted from the engine. Just a thought.
Try RPI's RP4 cam, new timing chain/gears and new followers if your running a manual. Also, if you have old heads on try get hold of some decent big valve heads. What carbs are you running?
regards, stephen