If it was me, I'd choose the squarest end and carefully explore for the centre, say with a centre square. Having centre-popped it, I'd chock the piece on the table of the drill-press so that the flanks were at right angles to the table surface. If you've got a big enough angle-plate, stand it on end on the drill-press table and clamp the piece into its 'vee' with a big G-clamp. Centre drill to enlarge the centre-pop. Then I'd transfer the piece to the lathe with the other end in the chuck and the tail-stock supporting the centred end, using a half-centre if you have one. Hopefully, the end in the chuck will be square enough for the chuck jaws to get a good grip - even if the piece only touches the face of the chuck in one place. Face off, leaving the smallest possible spigot in the middle, hopefully small enough to go through the hole in the chuck or face-plate. Marking-blue the end and mark out the holes to attach to the face-plate, remove from the lathe and drill and tap the holes. (If you're feeling really fussy, file off the spigot first.) Transfer to the face-plate and proceed according to John S' method. Having got the piece securely mounted on the face-plate, there might be some merit in facing off the now outer end and putting in a set of tapped holes in that end too.