i just put the motor im my 86 chevy and when i try to start it with the batery low i get smoke from the ground wire that goes from the batery to the fender the wire is almost gone now and i dont want it to burn up the new wire any idea on what causes this?the truck turns over really slow also  

the wire that goes from the battery to the fender, that is a small diameter wire? correct?
There should be a thicker diameter neg. wire that goes down and bolts to the engine block I think.(you tell me)
 if my assumptions are correct about the placement and diameter of grounds the reason the small wire at the fender is burning up is cause all the current from the starter motor is flowing through that small wire that is not designed to handle that much current. which would mean there is a connection problem at the thick ground wire probably where it bolts to the front of the engine.
(you will have to excuse me I can't remember off the top of my head how the ground wires are on that van)
 Let me know if that helps, If the ground set up is different on your van than I am assuming let me know how it is and we can adjust our strategy.
 Also, if you have a voltmeter I can walk you through some very cool ways to pinpoint the source of your bad connection. (voltage drop testing)
Some other considerations would be. Does the engine spin over as it should if you turn it by hand. Is the starter drawing excesive current. I betcha its a connection issue though.  

its a 95 chevy 350 van motor in a 86 chevy k10 4x4 and im gunna check that today i bet your right it was painted where that wire hooks up  

I had the same problem on my '86 Blazer and it was the same solution that you suggested-no ground or loose connection at the block. Small wire connecting to frame would smoke and became extremely HOT. Made a good connection at block and problem was solved.  

Make sure your ground going to the block or alt. bracket in some cases is tight, you might just want to replace with a new ground wire, this should fix the problem