Not tried the Alpha in a S-2A but it is fine in an S-2C. I'm just over 6ft and have only donked my head once or twice when sitting in the back of the S-2C when my straps weren't particularly tight.
Personally I went with an HGU-55 in the end, which thankfully made it through customs without any trouble. I think the final damage was about £785 all in.