[AZKen]

In general, if you don't know the size of bolts, you measure the thread diameter of the threaded hole or the clearance hole. Old trucks just used a few fractional sizes and mostly coarse threads. Read up on thread sizes and get some calipers and a thread gauge. The type of work you are doing makes you a genuine mechanic, not a tinker-dinker. So get some of the tools I mention and learn screw/bolt sizes. The lengths are determined by how much length it takes to go thru (or into) all the parts, including washers and nuts. If there are blind threaded holes, stick a screw driver in there are see how deep it is. Then back off some. The area you are in now will be 5/16-18 , 3/8-16, 1/2-13. Maybe 7/16-14. This is basic stuff you need to be able to figure out. It is not necessary to ask and wait for that info. You can always try a bolt in a hole and see if it threads in, then take it to the store to get a correct length and etc.

It's good to clean out (chase) the threads gently with the correct tap ,and oil, if they are old, dirty, rusty. Clean gunk off threads with brake cleaner or carb cleaner or soap and water. Nice to have a good tap and die set which will include that thread gauge. Find a set with hex dies not round.

It is great to ask all about your truck and how to do this and that. We will help you do anything. Not talking down to you, just teaching you that hardware is hardware. It's THE basic part of "mechanican", if that's a word. You have all the parts, just look at them and measure the holes. Clearance holes are the one's with no threads that the bolt goes thru. The correct bolt will be the nearest fraction less than the hole size....to leave a little "clearance" for movement and tolerance buildup. This will be confirmed by the threads it goes into.

Be sure there are no cross members or obstacles that will prevent installing motor and trans together. If not, it's a good idea to attach together.

The fork is "clipped" to the bell housing and loosely holds the throwout. You will have to have the motor stationary and the tranny slung or sliding onto the motor. You will have to fiddle and diddle the trans shaft and throw out to gently insert shaft into, and thru, the throw out and into clutch spline. Rotate trans shaft by turning rear yoke as you enter, to mate splines. The throw out bearing may fall out and have to start over. The key is having those two big things oriented to be able to slide tranny without much effort. On the floor/floor jack or hanging on chains.
I assume you know you need to align clutch with a tool as you install it to flywheel.