Why don't you let a pro fix it? It'd pay for itself in a few tankfuls. Or if its been condemned (as in "you don't want to start from here..") is selling it on really a good idea!?
cheers, Ross K
p.s. forgot the useful part! Drive till empty is fine to empty the tank, but do expect residual pressure in there and any pipes you disconnect. Disconnect nothing in the garage, do it outside and well away from drains (lpg is heavier than air ..)
Electrics should just strip out easy, connections to coil battery etc just come off. With one exception - somwhere it must diable the petrol system. On a carb engine, there will be a solenoid in the petrol line. Remove it and link the pipes back together properly. On an injection system it will be interrupting the injectors somewhere ... anywhere ...
There might be a box or two with a spider web of cables plugged between the original injector wiring and the injectors themselves. Removal is easy.
-Or- there might be two boxes with a spider web cut and spliced into the original loom. You should be able to pair up the wires e.g blue goes with blue/black, red with red/black etc. You'd need to cut and link these pairs to restore normal injector supply.
-Or- there might be a relay or a pair of yellow wires out of the gas ECU or suchlike, which somewhere (maybe around the head, maybe on the bulkhead, maybe under the drivers seat) will interrupt the brown-striped-orange supply to all injectors. You need to cut and reconnect this together.
Good Luck.