Difference in 3 speed manual transmissions Truck/car ?????

The earlier 3 speeds did not have provisions to mount a floor shift. Column shift only. And they do not have synchro in first gear. Way back there were after market floorshift conversion kits but they were not very good.

Since you already have a floor hump and other bits in hand, you should consider finding a newer A-230 as previously mentioned by Alaskan. The floor shifter bolts up like a regular 4 speed. These were the stsndard A body trans starting in 1970 or so. If was even standard on 340 Dusters.