If you have the heads cut but not welded, you could jig/clamp the head halves together and then disassemble the rocker assembly to use only the rocker shaft and the two rocker shaft mounts (not as picky for distance between rocker mounts) to mark/stake the rocker arms as indicated in the better half manual.
If the heads are welded, you could make a jig to locate the pair of cylinder heads with a larger space between them, but keeping the head bolts for each head aligned on their respective axises and equally spaced. To account for the larger distance between the posts you would (may? depends on the distance spanned) have to source a straight shaft or tube that you could use to ensure the rocker shaft mounts are following a parallel axis to each other and the heads.
