You can put the Volvo B234 16 valve cylinder head on your 2.3 lima block - it flows very well, somewhere between a well-ported iron head and an ARCA head, and the powerband is phenomenal. Myself and a few others have running examples. There's a Yahoo group called "2300-16V" that's dedicated to discussion of this head swap, and some discussion of the swap often occurs on turboford.net.

When you say "2012 Mustang" you're talking about a tube-frame racecar chassis that you're campaigning this year, and not a 2012 model year Ford, right?