I wouldn't put a lead acid battery on it's side - even a sealed one. If it's just extra starting power you need why not install a higher capacity main battery like a 664? The other option is a motor cycle battery beside the main one under the bonnet in the normal place. That way the extra wiring only need be about a foot long. That'll give a little extra zing and in effect be a permanently attached jump battery. (Wouldn't run the car on a bike battery on its own though unless you want to smell boiling acid.)