i've done this Steve...just use your intake gasket and flop it to the head. You're not going to gain anything though and porting is pointless unless your cam is bigger than about .490 where the flow stalls out.
They will support 400 hp stock. I think the X, J, O, And '73's were all the same casting and were between 72 -74 cc's.The early 340's were rated at 63 cc's and 10.5:1 with X heads by the factory but that was so they could be milled to 63 cc's and be built to 10.5 :1 to be NHRA legal. I've cc'd a few and the smallest chamber i found was about 72 cc. Stock they were around 9.5:1 and that's right on the ragged edge for iron heads on the street...so watch it and watch out.