70 Duster--conv. to 73-76 front disc??

All 1970 Duster 340s came with the Kelsey Hayes 4 piston disc brakes as standard equipment from the factory. My brother sold these cars new in 1970 and I purchased a new 1970 Duster 340. The 1970 Plymouth Duster sales brochure documents this well.

So just to get this clear, this gentleman already has the factory SBC K-H discs on his 1970 Duster 340, not drums.
Plymouth did resort to 10" drums on the 1971 Duster 340s as standard equipment, with the K-H discs as optional.

As far as having no brakes with the current factory SBC - Kelsey Hayes setup, have you considered having your calipers rebuilt with the stainless sleeves? Also, Firm Feel sells great upgraded disc brake pads for this setup.
Rotors are available from numerous sources, including slotted / drilled SBC rotors from Summit for $79 each. Raybestos, Wagner and Centric all offer factory replacement rotors from $39.99 to $92 each.

I have a 1971 340 Duster with the factory K-H disc setup and it stops great.
Check the adjustment on your rear brake shoes and the condition of your rear drums. New Drums are available in the 10 X 1.75" size from the same manufacturers listed above. Many times if your rear brakes are out of adjustment, the car WILL NOT STOP well.

If your car sits for extended periods of time, many times the pistons in the calipers will not move freely in the piston bore(rust build-up in the bore) or in a worse case, seize up. Brake fluid is hydroscopic, meaning it absorbs moisture and that moisture travels throughout your brake system. A hone cleanup of the piston bores and new seals ccure the rust buildup in the bore. The factory service manual describes all this in great detail with great large drawings of the caliper, and the entire front brakes.

The stainless steel upgrade is a permanent piston bore fix and upgrade.

I'm like you; I do not wish to move to the BBC, and I have found that there are many SBC wheels now available.

With some work, your SBC disc setup can work great, trust me.

Good luck with your brakes and let us know what you decide.