Never heard of an F7 being anything other than an engine. Or an f-stop for a film camera. I'll check in the "Stuff Box" to see if I have any of the longer driveshafts. Don't know that I do, but if I do, they are yours. I may also have some "extension couplings" that mount to the ends of the driveshaft at the motor or on the truck tower, you may be able to use these also, and get by using shorter driveshafts. If I find anything I'll PM you for your address and send whatever I find for you to use.
I had to do a similar project when I re-motored a couple of my DD40's and my DD35's(slugs). I ended up using extensions as I couldn't find driveshafts long enough. You may be able to use the driveshafts from a GP30, GP35, or GP40 also. Never tried, but being a two-axle truck, with engines of similar lengths, it may be possible. Like I said, I've never tried.
Measure, somehow, what you have, and what you need to give an idea of the length I'm looking to find.
Do you need the hex shafts(like on the Atlas and Atlas/Roco models) or the "ball and T" type(like on many Athrean engines)?
Bob