Yup, you can do it. It is an easy way to upgrade. The one thing you might have to do is take the core support, the radiator & front driverside inner wheel well from the 74 to make the PS work. If you look at the 68 & 74 side by side, the 74 will have a different wheel well to clear the rag joint on the steering shaft. The core support & radiator are different to clear the length of the PS box. Past that, everything will bolt up exactly the same. No changes needed...unless you have changed trannys in either, then you will have to either cut tunnel holes or weld them up or swap the tranny. It is a pretty straight forward deal, pull one of, put one on.
This would also be a good time to put new body mounts on. When you do go to swap the tubs, make sure that brace the bodies. And by this I mean brace the front (engine box) to the rear (bed). Take anthing rigid that you have and before you lift them off, bolt them side to side right behind the door pillars and then again front to back in the door openings. If you don't do this, you run the chance of tweaking the body and then you will have trouble getting doors & body panels to align correctly. Once you have them sturdy & supported in all directions, the best way is to either have 1 person at each corner and lift or to use straps under the wheel openings & under the doors and use a hoist of some kind. The bodies aren't real heavy, 3 people can lift them, but 4 is much better.
