My advice is to keep the low end power where you need it. iow, don't go crazy with the cam. A 256/256 Schneider Cam would be a great upgrade. Deck the block to get it near zero, then find a 78 or later head and have it milled to get the compression to somewhere between 9:1 and 9.5:1. Where are you, what's your elevation? What year is the 200 you have? You're going to want to put a Duraspark 2 distributor in with an HEI or MSD module for the ignition system. For the carb, I've had pretty good luck with my 300 YFA knock off so far. A Carter RBS from a 250 or a YF or YFA from a 240, 250 or 300 will do you fine.
If you like the 3 on the tree you're going to want to upgrade to a 3.03 toploader with the tailshaft for an econoline. I have one out of my 64 laying here wasting away. As for the rear end, from 64-67 Ford offered a 9" rear end in "heavy duty", falcon and super van options. It's a direct swap but you'll need the driveshaft or to have a new one made. They can be hard to find but they are out there, Vic La Fontaine in SoCal has several. As for the the ratio with a 3 speed, if you want to drive it on the highway at all I'd recommend 3:1, if it's mostly around town you could go as low as 3.5:1, but you will be screaming down the highway @ 60
The 200 itself will bolt right up to what you already have, but it will be sitting on the 61-64 sheet metal engine hangers. Some say they aren't sufficient for a built 200 b/c the 61-63 frames are thinner than the 64 and the 65-67 frames. But I think it'll be fine if everything is in good condition. They way the same, just a little more torque.