“…….and debating how long the 8.25 rear might last behind the 440, if only street driven, once we add a Trac lok…….”

Just my but I think it will probably be pretty short lived with a 440, even with reasonably sized street tires. Personally I think I would dig up an 8 ¾ or 9 ¼ before putting any time or effort into the 8 ¼.

If you can’t find a truck axle, the 76-78 full sized Chryslers (Newports and New Yorkers) are exactly the same width as the D100s of the era you’re working on, even take the same size brakes and drums that the factory used when they put them under the D100s. The only real difference is the spring perches are on the bottom of the axle instead of the top. I’m not sure if the perch spacing is the same between trucks and cars but if it is leaving them where they are is good for a 6 inch rear drop. My experience with these is that they are not in high demand and usually sell pretty cheap and you might even get lucky and find one with a Trac Loc.