hughes is correct in that the early 340 pistons stick out of the deck...

so if your engine has been rebuilt or is not stock..you need to check the pistons to see if they are above the deck at tdc..

if they are...you will need thicker head gaskets to maintain piston to head clearances...

so if your pistons are out .008 ...then get a head gasket around .055...you got .047 clearance..