Lift Springs/'64 T'all suspension questions

[MikeIIDC@domain.elided]

What kind of info does a company need to make springs for a vehicle?  I
have had some very good  recommendations of places to make new springs
for my Travelall, but they are all far away.  Are there any special
measurements they need that I shouldn't trust myself to take?  Are there
standard books that will have measurements for all vehicles or
something?  I am hoping to add a little bit of a lift when I get the
springs made-does this complicate things much?
Also, what is an "add-a-leaf" spring and what is an "overload" spring?
Advantages/disadvantages?
If I have heavier springs made for my rig, is this part of what it takes
to make it a higher rating?
In other words, what is the difference between a 1/2, a 3/4, and a 1
ton?  Heavier axles and springs-perhaps a heavier tranny for some, but I
don't think this would matter in  '64 T'all, would it?