If you choose to convert to 4wd the frame will be significantly elevated from 2wd configuration. A front differential means that the frame has to be raised to clear, period. Will it fit? Heck yes, you'll make it fit! The choices are what type of suspension are you looking at?

For me, I'd go with a long arm suspension with coils in front. Wheel & tire size will drive your true ground clearance, so you need to decide what you're building? Do you want something that's truly trail ready, or are you just looking for 4wd for cold climate, snow driving on moderate roads? It makes a huge difference in approach. Decide up front is this just a driver, wanting 4wd to be able to handle a big snow, or are you wanting an off-road rig.

Honestly, I don't think a '41 Truck would be my choice for conversion to 4wd. There's too many other candidates out there that make more sense. Fat fenders and running boards don't equate well to sitting tall on a set of 35's or bigger. Still, it's your truck....