I think you better do more than keep an eye out for an engine. The suggestions graygander gave you will get you as far as setting the engine in the frame. The wiring harnesses have several differences and these are systems that you are going to have to educate yourself about.
Your '67 has manual steering and manual drum brakes?
You're adding a lot of weight that is going to make it a lot harder to steer and to stop. You'll have to use a Hydro-vac for the brakes and that's a conversion project in itself.
The only sensible way to do this conversion would be to buy a complete running truck that you can drive around for a few days. That way you can make sure everything is working that you are going to need and you are going to need everything.
Finding a good unmolested 6.2 truck is going to be the search of a lifetime. Half that you will find have been molested and the other half will have the diesel replaced with a gas engine years ago. Finding the engine alone would be the beginning of a big head ache unless you are one very smart and resourceful cookie.
If you really want to do it, I'd shoot for an early '90s 6.5 Turbo at a minimum. You want this conversion to be interesting, don't you? Problem with that is that you also have to get a '70s or '80s truck for the Disc brake/steering parts. The 6.2s were easily able to tear up the automatics of the time. They were probably the TH400, if I remember. The 6.5 might have the 4R80 with overdrive.
This is starting to sound like a great truck.
