If you do a set of aluminum closed chambered heads with a piston that sits flush with the deck, you can use a standard .040 compressed thickness head gasket. This combination will get you a quench build which will allow you to run a little more compression on pump gas and make a little bit better power. An aluminum headed quench built 440 with a mild cam can do 10.5 compression on pump gas where an open chambered 906 iron headed 440 would top out in the low 9's compression range.

This is the preferred way to setup your engine these days, but by no means the only way. Another consideration is cylinder heads. If you have a junker set of 906's that need to be rebuilt, then buying a set of aluminum aftermarket heads like the edelbrocks or 440source's starts to look really attractive because the price to rebuild a set of stockers is not cheap, not to mention the benefits of the extra flow and improved cylinder head design and materials of the aluminum heads. If you have a good set of iron heads already done up, then you can look at a set of dome pistons you can grind for quench.