Steering Coupler ?

Don't think that will work. Problem is attaching a coupler like that to the shaft. The shaft end is not splined at the coupler, looks like that coupler either uses a cross bolt (shaft would possibly have to be cut to correct length and drilled for the bolt) or it uses some set screws and jamb nuts which I wouldn't trust my life on. You stated you planned on going to power steering in the future. The shaft diameter and/or splines are different on a power steering box IIRC. Plus the shaft is shorter on a power steering column. Best bet would be keep what you got for now and then trade your manual shaft and coupler for a power shaft and coupler when you change over. Lots of guys here on FABO are looking for manual shafts to convert the other way.

I did a coupler conversion on my Barracuda to make room for headers but that particular set up is no longer available from the manufacturer. They do offer a Mopar coupler similar to the one you posted but I have no knowledge of the install. Check out Flaming River's site. A thread on my car and that coupling mod can be found at "El Mongrel" '65 Formula S Restomod if you want to take a gander.