Need help with 1975 Duster /6 to 440 conversion

It's not hard to find 8-3/4 rears, but it not too easy to find them already set-up (or original) for an A-body. If you hunt for a Ford 9" I'd try looking for one in a Fairlane or big old mustang, like 70-73. I have one in my '69. A driveshaft shop can make you up a custom shaft. You can get the parts you need easily, and you are likely going to want a bigger than stock diameter for strength anyway. As far as the motor goes, IIWY, I'd just spec a Road Runner (or 440 magnum) hydraulic cam in and run one of the new Holley 750's. Go vacuum secondary with that cam for simplicity, or if you decide you want a beast, a Purple Shaft .509 will fit with little effort and use the double pumper 750 carb. Aluminum radiators are very common now and there are options available that direct fit. Or if you can grab one easily, i'd just get a 26" unit for a "C" body, as it will do the job easily. TTI would be my choice for the headers, but they are $$. Good luck, sounds like a nice pro-street car, and could be a super sleeper if you do it right. HTH, Lefty71