Well if I had to do it over again I would have put a small block in my car and kept the big block for a drag car. When I first built my car it was going to be your typical Camaro that runs a decent quarter mile time, but then I really started to like the whole "pro-touring" theme, so I went that way. How much does the big block hurt? I don't know but it sure isn't helping the handling of the car. But it still handles pretty good, I think.
My ZZ502, with the crate it came in, was 642lbs. I think the crate more than makes up for oil, distributor, and carb. That was lighter than I expected.
I believe this is about the same as an all iron SB. I'm sure an all iron BB would tip the scales at 750 lbs plus.
I'm sure you could achieve neutral 50/50 ballance with an al. headed big block, you would just have to be very conscious of other things on the front end. Like relocating the battery to the trunk, fiberglass hood, and possibly fiberglass inner fenders/bumper, that kind of thing. It may even out when you add the weight for a roll cage/frame connectors and such to the back-half. Add a lighter aftermarket subframe, suspension, and brake components, and you should be there.
If you kept the total front end weight the same as a "typical" SB, you should be able to use the same high performance spring rates that are common now.
You could get into setting the engine/trans back a bit, but I don't think that would be worth the work - it could be accomplished by other means.