The 90 degree connector is attached to the axle with a stud that is integral with the 90 degree part. Be careful undoing it to preserve the 90 plus stud. Scrap the nut (either flame or grinder) if it doesn't turn easily. I have seen the 90 connectors both with and without thick washers and thin rubbers. The total thickness is limited by the stud length. If you can't get the 90 to snug down and sit still on the axle at a reasonable torque consider making a stepped washer to bush the hole in the axle casting (or even a small sleeve to fit between the stud and the axle to hold things still).
The studs on your links will be tapered.
Carb, if you are working a 216 the Carter W-1 is available rebuilt aftermarket or eBay to rebuild. If you have a later 235 or 261, they probably came from the factory with a Rochester B that was supplied in several base sizes and venturi diameters also different air cleaner attachments. Try to get the right style rather than adapting. Another option is a brand new Zenith 228, not cheap but 100% new. This is very similar to the carb used on the C60X.
http://www.zenithfuelsystems.com/downdraft_228.htm
Seats and mountings, just keep your eyes open. If you find a seat that is solid but without the springs in the back, grab it. You can buy extension springs of correct wire diameter and OD as 30" lengths that you can cut and bend your own ends onto to replace the seatback springs.
Bob maintains that it is impossible to restore a truck without having another to study and rob parts from. It is often cheaper to buy a complete truck that has quite a few parts that you need than to try to find and buy the parts separately. You will also then have either a source for future parts needs or trading material to get things you need from others.