Warm air is less dense. removing the heater will cause the air charge to be more dense, and will require more fuel.
If you want, port the EGR valve to flow a little more, so you will be running more inert gas in your intake charge.
What kind of #s are you getting? I have a 455 with a 7.4 TBI unit, and it gets about 18. You could consider ditching the stock computer and going megasquirt so you can play with the a/f ratios.
Also, is the stove installed on you exhaust manifold? I provides hot air (less dense) so it will hurt fuel economy if it is not installed.