A '78 is an awful lot heavier than an '83, The general concensus is that a 300 is not a good swap for a '78, it has to work too hard and the mileage gains are lost. On the other hand, I've never tried it and you can build up a 300 quite nicely if you know how so I would be really interested to see how it works out.
I think the biggest weight difference would be from the '78's cast iron driveline components. The body metal feels heavier(and there's more of it, and less rust.. haha) on the '78, and it probably is, but I figure it's worth only a couple hundred pounds at most. (I hope) My '83 has the heavier 9" already, that I swapped in last year.
I have a truck scale at my shop, now you have me interested in comparing their weights. I will weigh them both and post the results when I'm done. (I also have all kinds of tools, spare parts and junk in the back of the '83 adding extra weight, mostly because the rear window wont move anymore, as the body is settling so bad, it's screwing up the latch switch.)
Even so, on my way to work, I take secondary roads, at 40-50 mph,(my '83 is SCARY at high speeds!) and the 300 makes enough low speed torque to leave it in overdrive the whole time, even on hills. It's making 280-290 ft/lbs at about 1800 rpm. They're amazing truck engines. It will pull a trailer about twice as well as the 302 I used to have in there. (and the 302 was getting about 10 mpg)
And there's always this option, which is very appealing:
http://www.fordcummins.com/