Transmission weight has little to do with the equation. What matters most is final drive gear ratio and overdrive gear ratio. And the question you need to ask is what overdrive and final drive gear ratio can the Duke 2.5 actually still make enough power to maintain highway speeds using? And that is going to be a question of engine torque production and not so much about how much HP it makes.
The overdrive ratio of a 440T4/4T60, 4T60-E, and 4T65-E is 0.705:1. The common final drive ratios of these transmissions are 2.84-2.86, 2.93-2.97, 3.05-3.06, 3.29-3.33, and 3.69-3.73 depending on what gear ratio diff and chain sprocket ratios you are using.
I think it might be possible for a Duke to work with one of these overdrive transmissions if you are using a 3.69-3.73 overall final drive ratio since that would give you an overall ratio of 2.60-2.63:1 in 4th/OD gear. The reason I say this is because the Duke was used with a Muncie 4-speed manual transmission in 1984 that had a 3.32 final drive and a 4th gear ratio of 0.73:1 = an overall ratio of 2.42:1 in 4th gear. Keep in mind the automatic will drain more power from the engine than a manual will (due to power used up by running the hydraulic pump in the automatic among other things), which is why I would recommend using an overdrive automatic that has a 3.69-3.73 final drive ratio.
I think it is doable. But if you wanted to use a 4T60-E or 4T65-E, you'd need a stand-alone module/computer to run the transmission. The 440T4/4T60 will work with the stock Duke ECM.
OVERKILL IS UNDERRATED
Custom GM OBD1 & OBD2 Tuning | Engine Conversions & more | www.gmtuners.com