At the risk of sounding simple or being too obvious I'd ask if you are sure you don't have the ignition timing too far advanced? That'll definitely make it labor during cranking. After that I would see if you can get a hold of a borescope & take a look down the cylinders & check for damage as previously mentioned. A borescope would save you some of the hassles of tearing the heads off of it right away. If all looks well then I'd inject some oil into the cylinders as suggested & see if it losens up. But even then I'd be hoping no damages done already.