[Sizzler]

The biggest issue with the Shelby heads is their gigantic chamber volumes.

Nowadays, performance heads are designed with small chambers...72, 76 cc. Shelby chambers are in the 80's before any sort of cleanup. That size chamber will work, will work well if your engine is in the 500 cubic inch range, but generally speaking, are just too big otherwise. Some CJ heads for comparison sake (and the new Blue Thunder heads I hear) are in the low 60 cc range chamber size-wise.