Start by thinking through your answer to Grigg's first question, What do you want to have in the end?
The front track width on the '65 is enough wider than your '51 that even with offset wheels you'll have trouble fitting them under the fenders. Guys who use that IFS under an AD truck often narrow it first.
That's not real practical if you want to use the complete frame.
Also when using that IFS they usually stay at stock height, at least in part because of tire/fender interference. Dropped spindles are going to increase that problem.
Search for stories of guys using '73 to '87 IFS under an AD truck. The IFS is actually the same from '63 to '87. The later ones have 5-lugs and disk brakes which seems to be why they are more popular.
The rear is more forgiving, but the '65 is still a little wide and will limit both your wheel choices and how low you can go.
Which rear suspension does it have? Leaf springs or trailing arms?
You don't say how long either truck is. A 1/2-ton AD should be 116 stock and often an inch or so longer lowered. I don't recall the length of a '65 but I think the short-box is shorter and I know the long box is longer.
