225 Slant 6 Head Gasket Needed

Ditto. I read that some slantsix.org guys got together and had a batch custom-made several years ago. But first evaluate your block. I read that many are corroded so bad that one can't mill a flat surface. If you strike out, don't fret since a used steel slant can be had very cheap or free, and I think bolts right up (~1967 or earlier, otherwise need crank adapter ring).

Another aluminum block interest is a hot rod magazine article you can find on-line. They ran drag tests with an aluminum block vs cast-iron. Surprisingly, the lighter block weight didn't give faster times. They attributed that to the cast-iron being stiffer, preventing blow-by or such. Hardly definitive, but interesting read.