I love the original F-1. It is absolutely a creature of its time.
If you want to keep the original mill, you can wrestle a little extra power and efficiency out of that engine with some higher compression pistons. The originals will be pretty soft at about 6.0-6.5:1 CR, so the ignition event is more of a "poof" than a "pop." You can find domed pistons that run in a stock block under stock heads that will bring that up to as much as 8.0:1. Pump gas will run with no problem in that.
Overhead valve heads are not that hard to find either, but obviously that will bring in some replumbing on the exhaust and intake. And if you go that route, you could seriously modernize by converting to EFI at that point.
If you don't want to keep that flattie, post that rascal on Craigslist and hope like hell your phone doesn't catch fire from all the interest. There are avid old-school rodders who would absolutely love to get their mitts on an original flathead.
Repowering with a 2.3 Lima engine keeps it all Ford (if that's important to you, I know there are Aussies who will come to blows over the GM vs. Ford debate), keeps the power comparable to what's already aboard, and has lots of room for further development. The 2.3 Lima made over 170hp in the Mustang SVO with no mods in the bottom end - nice! And it makes decent torque for a motor its size, too, and even more if you go with the Lima 2.5. That's a pretty good truck engine if you're not going to be too demanding. The Mazda-sourced Duratec 25 was available in Australian-market T6 Rangers starting in 2010, you may be able to find one in decent shape in a wrecking yard, along with the transmission and axle to go with it. That would give you modern driveline hardware and running speeds while keeping it all kinda-sorta in the Ford family.
This would be a fun project. I think modding on old body-on-frame trucks has GOT to be more fun (read: less hassle) than trying to mod newer spaceframe designs.
[edit]
Hey, I just found this:
http://www.ford-trucks.com/articles/...rential-swaps/
Among other things, it lists other vehicles that could provide an axle of the correct width. Looking for different ratios, it may be easiest just to find one of these in a wrecking yard and put a whole new rear end under your truck, if the ratio is more in line with your needs.