C69 stock head installation - left vs right

[Ziggster]

I have short block back from the machinist and wish to temporarily attach the stock cast heads. He did blast them, and they are crack free, so au wish to install them. However, when I purchased the engine it was already disassembled, so I have no idea which head goes where. Does it matter, and if so, how do I discern the left from the right? Thanks.