Sounds like this is a combination for a Mopar race block which came rough bored to 3.91 or if you are lucky a 318 with thick cylinder walls if if would sonic check to that thickness. I found the following on another Mopar web site.
"It is an 'A' engine crankshaft, surface hardened, forged 1053 steel small block crank,featuring radiused fillets at main and rod journals, eight bolt flanges,straightness checked (318 mains). This is Part # P4120312. This is a 3.45" stroke."
This would give you ~ 355 cubes.
Here is a link also to another article with the crank part numbers referenced.
http://books.google.com/books?id=S7...ed=0CBkQ6AEwATge#v=onepage&q=p4120312&f=false
