My buddy gave me a power steering gearbox that came out of a 75 a-body. He said it is tight and no leaks. If I use it in my 70 coronet, what do I need? Just a pitman and idler arm for a 74 Charger?

just the pitman.
the idler should be the one for your car.

when I did the swap for my 65 coronet, all I changed out was the pitman arm and the hose piece to get the 74 and up to work.

I don't know why, but I always thought the pitman and idler had to be matched for the same year?

I think you are thinking of stuff like fast ratio. I think in those cases they have to match.

but year to year the difference makes it impossible to match.
as I recall, in some cases how they bolt up differs so it wouldn't make sense.

Thanks! What hose piece did you buy-74 charger?

I swaped it off my old one.
the pressure line fitting.