75 Duster Bumpers - Tuck in without extensions?

No.
With some modification, you can attach the fronts, but the rears, even with the brackets seem to be a no-go.

The problem is, where the stock early brackets attach, there is nothing but a big gaping hole in the 74-up cars where the bumper shock goes into.

You could do what vintage BMW owners do though.
Remove the filler panel, drill holes in the bumper shocks to drain the fluid, then push the bumpers in.