As said by others above, i wouldn't remove the heater.... this is of course presuming that it actually does demist the windscreen as intended... it could be you keep the heater and add a computer fan to underdash to blast air over windscreen. ) this adds very little weight. The heat given off by the matrix is taken from the head/block, the extra volume of the matrix and the heater hose increases the latent mass of the water/coolant, and therefore acts as a heatsink by being able to draw ( hold) more heat before the block heats too quickly. ( so radiant heat loss plus bigger heat sink = cooler engine)
As you are in the ground up phase, it is well worth checking the condition of your radiator core... as if it is filled with accumulated gunk or has many fins damaged then problems arise. A number of Historic racers, actually disconnect or tape up the heater matrix/auxilliary radiator ( albeit plumbed in the front of the engine bay to get the wind) on cold days as there engine struggles to get up to temp.