My off-the-cuff answer is to reinstall the new flange in the same orientation as the original, if known. The splining and flange u-joint cup spots should match up with your original flange, but never hurts to check and confirm before install.
If original alignment not known, then we need to look for info on the proper "phasing" of the driveshaft flanges/yokes, which IIRC is listed in the Shop Manual. I'd have to go look to confirm where specifically. You could most likely use the
1955 Models (Second Series) Shop manual as that's going to have the same info applicable to your '55 First Series open driveline.
