the two piece is thinner than the one piece. I had to use the two piece on my XB3M 345HP Ford crate motor , with an early front cover. The one piece would not clear the front cover. timing chains are different as well. the double roller chains can hit the block around the crank gear. look very closely. i have three new chains hanging on the wall. manufacturer's make a slightly different chain and the machining of the block definitely affects which chain you can use.
outlaw66