Anyone out there know of either OEM or other replacement sealed beam headlights for the 1972-1973 DT/RT?

I'd go for one of the aftermarket headlights with the replacement bulb facility, although you might need to change the wiring pigtail.

The original DT1 headlight did, in fact, come with an APF bulb which could be replaced, and these do turn up occasionally. 

In the past I have obtained 5 3/4" headlight lens that fitted H4 bulbs from Octane Lighting. I believe they also sell 35/35 watt H4 bulbs. The headlight lens can be fitted into the headlight ring with "W" clips.