There should be a few differences...one would be the tranny tunnel(floor). The Shifter holes for the manual vs. the automatic will be in different locations. You may be able to fabricate. You would also have to get a new console trim piece. Second the driveshaft will need to be out of an automatic vette......
Can you use any old 700r4? No......you will have to have the tailshaft from a vette in order to line up with the tourqe bar(that connects the tranny to the rear end).....if you found a regular 700r4 and a corvette tailshaft..you may be in business...but I would definately have it rebuilt first with the corvette servo and high performance parts.
Just my 2 cents worth but, personally, I would keep the 4+3.....
but the grass is always greener on the other side( have automatic)
Chris