The frames are basically the same. There is a difference in the center crossmember. One is welded in and the other is riveted in. I'm assuming you are talking about passenger cars. Pickups are another story.
The V-8 club's 41-48 Ford book indicates the 41-48 frames have the same dimensions.. The 41 uses a narrower front axle to compensate for the fact the front fenders are narrower and pinch inward at the front.