If you don't have a really nice set of jumper cables, it will do what you are saying.  I use really heavy duty thick jumper cables.
Do you have a battery charger?  You say you jumped it on your starter....wasa it directly on the positive bolt on solenoid?  Still, if you have a so-so set of jumpers, aint gonna do well.
You say it will turn over slowly....well, that is a start...it aint the tranny pump or it will just go clunk one time and freeze up. 
Do what vile said....get a socket set and turn the crank over with the 5/8" socket manually a few times to see of free it is.  
You should not need a high torque starter just because you changed transmissions. 
I believe it is still your battery.  
SgtDel
aka "Andy"
www.delbridge.net
1979 Big 10 383 stroker
1991 Blu 'Burb 3/4 ton 4x4
Edited by: SgtDel   at: 4/8/07 1:01 pm
 at: 4/8/07 1:01 pm