It would be helpful if we knew exactly which model of transmission you have, and which gears you need. There should be numbers cast on the side of the transmission case. And if you can post pictures of your damaged parts, we may be able to match them.
From the research I have done previously I am guessing only he's talking about one of two transmissions, the rock crusher (markings T8D-1A or 4TA/4Tb) or the synchro silent 4 speed (markings 4TH or T98).
No synchro on the rock crusher... and if you are willing to wait till I finish my T5 swap I have a whole rock crusher verified working (I'm driving on it right now) I'd let go for a song and a dance :p
Are you building a museum piece or a practical drivable vehicle?
I saw you visitor message, and just have to double check before discussing the T8 seriously. No synchros, no overdrive, and low is geared for lower than towing - clear down at rock crawling. The stock T8 is undesired, nobody keeps them they all end up in salvage. They weigh 175 pounds (85 kilos), shipping to Germany would be a mint. If you are going to all the effort to put this back together as a usable vehicle I would look for a T-5 and the pieces and parts to convert it over. The T-5 weighs 75 pounds (34 kilos) and is sold internationally (the S-10 donor truck is sold in Brazil and USA but not Euro that I know of).
My focus has been on US available transmissions. I did some digging to see if the Chevy S10 pickup version T5 is available outside the US, but so far it doesn't look promising.
Time for some of the European friends to step up and chime in - what did you guys find for options?
As fifty-two_f1 said, a T98 is NOT going to fit in your F1 without swapping the transmission crossmember, master cylinder, and pedal assembly to one out of an F2 - F6 truck. The crossmembers are riveted in place and changing them is no small task - and that's after you've found a T98 to put in. They aren't so easy to find. In my opinion, there are better choices. I'd either go with a T5 swap or a stock light duty 3-speed.
T98s from later trucks are fairly easy to find. Starting with the 1953 trucks all the four speeds were T98s. This stayed true through the 60s. I am not sure what differences in the T98s there are between years.
The 48-52 T98s have the bellhousing that will bolt up to the flathead 6s and 8s and the 215 ohv. The 53-56 T98s have a different mount in the front and have no rear mount, they hang from the bellhousing..