Is this a 1973-75 O'DAY 20, or a 1976-78? In other words, masthead rig or fractional rig?
If the boat is a 1976-78 and has the fractional (taller) rig, then the spreaders do rake aft but the brackets should prevent them from pivoting too far aft. They swing forward when the mast is unstepped and will theoretically self-align to the shroud angle when the mast is stepped. The same spreader bracket is used on the Mariner.
On the masthead rig (1973-75) I think the spreaders should be more or less in line with the chainplates (ie: shroud should not be deflected fore or aft by spreaders). I'd give Rudy a call at D&R Marine to get the correct length of the spreaders, then measure to be sure that yours are not too long. If your mast is original and not modified, the spreader bases would be fixed, that is the spreader tubes fit into a tubular base with no way to pivot fore & aft (based on pictures that I have of those model years). It appears that the spreaders on most 20s are nearly straight across, not angled back.....so something doesn't sound right.
Generally, all spreaders should be positioned so that the spreader angles up slightly from mast to tip (outer end higher than inner) and roughly bisecting the angle of the shroud running over it.