What kind of truck frame can I put under a 1946-47 3/4 tonne Chev Pickup? 
(ie., S15 or S10)

I hear you saying you have 1947 Chev truck body and want to drop them
on a different chassis? If this is a real restoration you'd want to
stick with the stock frame. If this is a street rod or if the existing
frame is shot, that is something else. If you have enough of the
existing frame to get measurements off of, do that. This site shows
how many different measurements are possisble:
http://www.chuckschevytruckpages.com/framespecs.html (daunting eh?) I
have a Chervolet Parts Manual which covers 1929-1955. All 3/4 ton
chassis except the Double Duty has the same frame for 1947-1953 (this
probably has 3600 chrome number on the sides of the hood.) This
expands what you can search for. There is lots of information out
there using Google.com and {"truck frame" chevy). Several are
http://www.autorust.com/ and
http://www.classicperform.com/tech_articles/c-notch/c-notch.htm. You
are starting a very lengthy project. Further research will be your
best friend.