Oh, it can be done.
Keep in mind, it will cost you. If the only reason for a swap is mileage gains then finacially speaking, that is one hell of a long term investment before you ever recover your cost.
If your engine is on its last leg and soon to be reworked or replaced, then a 350 swap would an understandable choice. You can almost buy parts at 7-11 for a chevy 350.
I like the idea myself. I am waiting patiently for the old girl to fall over and die but in no way am I rushing it.
When it does, a 350 swap will certainly be the approach I take.
If money is going to be flowing, might as well make it worth while and take a little power trip along the way.
BTW... when that does happen you will not find a LUCUS product anywhere near this truck. LOL