front disk brake conversion kit
Before you dive into a conversion and opening that can of worms, are you SURE your brakes are up to snuff and performing to their designed potential? Duster 340s had 10" drum brakes, and while they may not be world-beaters by todays standards, they should definitely be better than "not stop well"- they'll lock 'em up and darn near put you through the windshield when operating correctly.
Make sure your brake shoes are in good condition and adjusted properly, and that all brake hardware is present and intact. Make sure the system is bled properly. After 50 years, are you sure the master cylinder hasn't been replaced with one with an incorrect bore size? Brake hoses can also go soft over time, contributing to a "spongy" feel.
I have never had 10" drums feel inadequate in normal driving.