Andy,
The carbon fiber heaters that we sell have dimensions of 32 x 20 x1". So if clearance is a problem, you need to consider the size. As far as how far you need to be away from them, there is no problem in this respect. The radiation from them is a low density. We recommend leaving a 1/2" space behind them. Power is 300 W/unit whether 120 v or 240 volt.
Dieter