The angle needed can vary, depending on the frame and body clearance. On my '37 Oze, the engine is set dead level. You couldn't get the engine up more than a degree or so without the bell housing hitting the trans tunnel. My frame was supplied with the trans mounting plate welded on, so it wasn't practical to drop the tail of the transmission to get more angle.

To figure out how much rod adjustment is needed for 3 degrees, just multiply the vertical distance between the rod ends by .052 ( the sine of 3 degrees). For a 6 inch spacing, that's only 5/16 inch. Each rod end could move half that ( one in and the other out).