318's thicker ?

dont much understand either. {try not to chirp? ...WTF does that mean}
any way, no...318's were not made from 340/360 sand cores.
some, and I do mean some percentage around 25% maybe of the EARLY 67-68 318 block can be bored out to 4.00 inches and that is it. 7000 rpm?? Just get a 340 replacement block or an R-3 block. Better yet would be the 48 degree block if your really serious about spinnin some rpm's. best bet for low $ would be an early 360 block which really was made from a 340 sand core.