318 into a 340

i found this it may help. i also saw a few drawbacks on the poly engines. mainly part availibity or lack there of. check into it carefully before you jump into it. maybe someone else can advise you on this.

The 277-318 old style cylinder head is generally called a poly-sphere. Looking at the strange valve angle it appears to be a modified Hemi. It certainly isn't a wedge. The casting number is 2268341.