Things start to snowball around this point.
A SBC with the Novak adaptor is cheap and easy. A SBC with a 465 married 205 would be the next step up in my opinion. But, you will do new cross members, drive shafts, clutch linkage and exhaust work.
If you want to go overdrive. They all came married with driver side drop transfer cases. So a Ford D60 could be swapped in or a passenger side drop 208/241 could be bolted up. Then all the stuff mentioned above plus a hydraulic clutch set up. Along with probably a matching 8 lug rear axle.
A divorced overdrive is easier, but still more involved than a non overdrive due to clutch set ups.
I did a 2wd NV4500 on mine and love it. But, they are kind of fragile compared to the old T98 or SM420 or SM465.