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I have a 72 C10 chevy with a 5.9 and a nv4500 with stock spring only lowwerd it 3/4in.
When you put a 460 with a blower and a couple of carbs with AC that will weigh more than the 6.9.I would just put F250 springs the stearing box is the same up too the 1 tons.It will work just fine.
The spring have different load rateing the f250 Supercab are the same as f350 with AC.
I went through this when I put new springs in my truck when I put the perkins in it.
[but the spindles bearings, tie rods ect... ect is alot lighter duty.]
I have put a few 460 and 429 in F100 &F150 and haven't had any front end problems just keep every thing geased good.
i am planning to put a 3.9 cummins in mine when the 300 dies . a friend of mine has one in one of thesetrucks and regularly sees 20 mpg empty. and any mods done to a 12 valve can be done to these engines
Hell yeah, Do it ! Think of the gas mileage you'll get, do the 3.9L & you can just wave at these guys that have 351's & larger as you pass the fuel station & smile because you're getting more MPG..........lol
I did a 6.9 in a '75 F250 highboy. I put over 100K on that setup and never broke any components. Weight is the only issue you have. The engine does not have enough power to really break anything.