I will go on record and say the 990 Chevrolet casting s are not near as good as I used to always think they were, and that many people even now think they are. The lowly 049 Oval Port can easily be made to out perform them without much money invested. Almost all decent aftermarket Aluminum heads will be better, not becuase they are aluminum, but becuase of the port shape and flow charastics.
I had suggested the Canfield, but just look around and see there are others that are pretty good including the factory GM small aluminum heads.
I suggest to get a crank and rods if you can afford them that is not cast iron or cast steel. Get Eagle, Scat, BRC, Cat, or anything you can afford that will be better than the cast stuff.
I am ok with the cast factory crank up to the 600 hp levels. That is it. I am ok with factory steel crank up to 1,400 hp levels. It is stronger than most think. I am ok with good factory block up to that same 1,400 level when block has been studded and prepped.
If you wiat and look you can likley find good used aluminum heads reasonable where guys are stepping up and getting rid of their smaller stuff. As far as I am concerned the intake you already have and everything on top would be fine with either size engine.
If Curtis comes on here and disagrees with my thinking about the 990 follow his advice. I can post flow chart results of both the 990 and the 049 oval with same size valves and same minor port work and you can see them side by side.
Ed