Gotzoom on your C6 go to
http://www.smokemup.com/ auto math & for tire & gear ratio that should give all the spec. on the trans. and any good trans. shop can make it shift at 5000 with a little work as for the 4 speed it(would go with a TKO 5sp) will work with enough $$$$ that means every thing new from the back of the block to the yoke on the 9" + peddle ass. the AOD is not quite as bad a small block AOD & drive line change with a 4sp shifter belive shold do that one I belive.
I belive you are not going to hurt a C6 with a small block some are runing C4 & belive most stock trans shift about 3500-3800 the end of a stock cam torque range.
Sure there will be some one else out the that is better in formed than I on auto trans that will chime in.
Ken