A lumpy idle is caused by overlap. Overlap is the result of squeezing more duration in, which causes the intake profile and exhaust profile to overlap each other during the transition phase of the exhaust valve closing and the intake opening. You can reduce this overlap by going to a wider lobe separation. BUT, when you do that, the intake valve will stay open for a longer period of time after bottom dead center, which will bleed off valuable cylinder pressure. Unless, you alter the ramp profile to make the intake valve open slower, and close faster, instead of the the up slope and the down slope being the same number of degrees. There are all sorts of tricks you can do. But increasing duration will either pollute the incoming air charge with burnt exhaust during overlap, (there is more pressure in the exhaust during overlap on a turbo motor, so the air will go to the area of less pressure...the intake manifold) or will bleed off cylinder pressure at the end of the intake profile...both to some degree. Its unavoidable. I said this in a couple other threads, but typically, increasing duration 10 degrees will move the powerband up 300-500rpm. Dont get a cam that moves the powerband up to an area where the heads arent happy. If you do that youll end up with a really slow car. So do you have ported or aftermarket heads? If not, I wouldnt go more than 10-20 degrees beyond stock. A 204ish cam at the most would probably be ideal with stock heads. Around 215ish would be good for some fully ported irons. You can go even higher with some GN1's. Of course there are people who have built some really fast TR's who didnt quite follow this rule of thumb, but I dont think their combos are ideal.
Want the best way to do it? Get a cam that matches your heads. Strap the car to a dyno. Look at the curve and then get a converter that matches the curve. They say, "such and such degrees will put your powerband right here, and you will need converter X". Only way to know whats really going to happen is if you actually see whats happening in reality, and not some calculator.