As Martin and Paul indicate, Postwar truck blocks are a drop in replacement. They are fairly common and should not be too difficult to come by. Why not take up Martin on his offer?
Of course you want them with good bearings, gears, and wheels that don't wobble.
I don't know whether he has any left, but Grzyboski's Train store had new, old stock MPC F3 rear trucks at a very good price. If you are interested, you'll have to write to them. As far as I know, they do not list them on their site.
If you have binding issues, they can be addressed. I did so by adding shim washers where required. Unfortunately, the way I did it required wheels and gears to be removed and reinstalled. I don't know whether there is a way to get it done without removing them.