Regarding plugs, it is a good idea to have new plugs, but this is tied up with the lpg mixture's reduced conductivity compared with petrol mixture. Hence if you have an efi engine with the plugs gapped at 35 thou. then reduce the gap to 25 thou for a better spark. This is the same reason for why your leads need to be in tip top shape. If you haven't replaced yours ever, now is a good time to do it, and with a good quality set, like a genuine parts set which is only about £25, (sorry chaps, don't know about the rest of the world, but the genuine set does fit properly!). That goes for the distributor cap too. The genuine one is better than a good many after market ones.
Re SU carbs; yes they are admirably suited. You need a split supply pipe from the evaporator which feeds a mixer located in each of the inlet pipes from the air cleaner to the carbs. The operation is not quite so straight forward as on an efi system as to change over from petrol to gas, you firstly have to switch off the petrol pump, then let the engine run until it coughs or stalls, then switch the gas on and restart on gas. Obviously you can't just switch over to gas straight away as you will have gas and petrol mixture in the cylinders which won't burn and the engine will stop.
Therefore you do not start on petrol and change over to gas as a matter of course. You always start on gas as a matter of course if gas is what you are using, and you won't need to use petrol ever again.
Also with SU's, or Strombergs, you ideally need a vacuum piston lifting facility which involves a tapping in the dashpot chamber to connect to inlet vacuum via a solenoid which cuts in when the gas is on. This lifts the pistons out of the way and prevents them from going up and down as they normally do. This is not a necessity for running on gas, but is desirable as the pistons and jet needles are not wetted with fuel as they normally are and can wear out as a consequence. There has been some suggestion that rubber seals can dry out also if petrol is never used, resulting in flooding carbs when you put petrol throught them again, but you may or may not experience this, certainly if yours are in good condition, I would not worry too much.
When changing back from gas to petrol you have to switch the gas off and the petrol on, and wait for the float chambers to fill before starting. Otherwise, no problems.
In practice all these switching operations are undertaken with one switch, as on efi motors, but with three positions, petrol, off, gas. Use the off position when purging the system of petrol before switching to gas after the engine stalls.