The issue is the extra movement of an aluminum head on an iron block and also that the aluminum is softer. A steel shim can have a harder time sealing over time because of the movement (heat cycling) and can also brinell the aluminum. Only plus for the steel shim is it is cheap and it is thin. If the block has been decked it may be too thin for a shim all depending on where the piston sits.
The factory had a regular shim gasket and a better one used on some 427's (which is thicker too). Best practice today be cut the deck for put the piston at deck height to .005" below. Then use a .040" Fel pro 1020 head gasket or go with a set of Cometics MLS gaskets and be done with it. If you use the Cometics just make sure finish on the decks are 50 RA or better. I've used them on rougher without issue, but if you are starting from scratch do it right first time.
BTW: if you go with the Cometics may want to check out their top end kit. Is cheaper than buying a set of Cometic head gaskets by themselves and gets you intake, exhaust, distributor, and valve cover gaskets, plus a set of valve seals. PN: PRO1027T The head gaskets I am told are 4.30" x .040". I ordered this kit so will see first hand, should be here Saturday.
