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I'm not sure where you're located but it might be easier in the long run to sell your F-3 and buy a F-1. I'm in New Jersey and I'm constantly seeing great deals for Midwest trucks.
Vern, GreatNorthWoods, is the one member I can remember off hand that has done the conversion. He's got pictures in his gallery. Click on the "About Me" link:
Seriously, I'd wait and buy an F1 if that is what you want. You'll spend enough on the restoration let alone converting, you'll need top notch metal fabricating skills as well. And not to mention you will need to buy all F1 sheet metal (but a couple pieces) except the cab anyway!
What has to be done to convert a F3 to a F1.
And is it worth it
I have a 393w I want to put into it
Cut the frame just behind the cab with a 'Z' cut, take out 8 inches and weld the frame back together. I took the 8 inch frame section I cut out and welded it back to the inside of the framerail for extra strength. This reduces the wheelbase from 122 inches to 114. Next, cut 12 inches of frame off the rear (the F3 has more rear overhang than the F-1). Now the frame is exactly the same as an F-1. When the 8 inches comes out, the rearmost running board support bracket moves up so that F-1 running boards bolt on which means that you don't have to remove or replace running board brackets. An F-1 bed and rear fenders will bolt on. If you have good F-3 fenders up front you can keep them if you don't mind the extra wheel space cutout or you can replace them with F-1 fenders. You will want to replace the rear fenders with F-1s since they are wider than F-3 rear fenders. Actually, I like having a converted F-3 because the frame is thicker and heavier.
As to whether it is worth it or not...only you can answer that question. In my case, the truck I bought was in poor condition with no bed and the running boards and fenders were no good so all I had to start with was a cab, doors, and hood. I would have had to replace fenders, running boards, and bed anyway and F-3 parts (running boards and beds in particular) are a lot harder to find than F-1. I think if you are having a problem finding a nice F-1 to build then converting an F-2 or F-3 is a logical choice. They are usually cheaper than the F-1s and you can always sell off the good parts to finance buying the F-1 parts you need...
This is great info! The big trucks are a lot easier to find and usually cheaper from what I've seen lately. I just bought a F100 frame so I could downsize my F500 cause it was a great deal. If I didn't find a killer deal on a small frame, cutting down my 500 frame was next.
If I saw this, it might have been enough to make me rethink the small frame swap.