What you are planning to do is a BAD idea. Listen to what @morani said.
Other pipes are part of the emissions equipment. No petrol is allowed to evaporate and leave the tank (to the atmosphere). I am assuming you have a Euro II Bolan.
You asked what’s wrong with using a Freon canister as a petrol tank. Ok here are just two reasons. You will certainly be making new holes in the Freon canister and weling nipples and fittings for the line to the pump, then more welding for the tank filler nozzle plus cutting and welding for the emission tubings. All this exposure to heat will alter the metallurgy of the Freon canister, induce stresses in the structure also who knows what will happen to the seam joint of the canister as a result of the exposure to heat. Basically after all the welding is done you have no idea of what has happened to the structural integrity of the canister. The second reason is your mechanic (who will be doing your work) is not a designer. A mistake on his part may cause excessive pressures/leakage during use in this contraption of yours and may really make your soul fly high. Don't under estimate the distructive power of pressurized petrol. Find a good used pertol tank and stick to the original design. If your are suffering from repeated tank failure, investigate the cause. Certainly there are thousands of Bolans on the road not all of them are suffering form frequent tank failures.