[craggle]

I think it all depends on how many miles you intend to do in it.

I run my everyday runabout on LPG (4.6 Range Rover) and it cost £2K to convert to gas. This has paid for itself now as I used to do fairly high millage in it. It also has the option of going back to the standard petrol system if you run out.
If your Cobra only manages 4-5K miles a year then it would take a long time to make back the cost of converting it to gas.

Other things to consider are the size of the tank and where to put it, Lack of gas filling stations if you go for a long run, You can't take it to LeMans unless you use the ferry and a gas tank is made of far thicker material than a petrol tank so will weigh more.

Not trying to put you off, But think if it's really worth it.

Craig.