It will fit with a stock 84-88 G-body Crossmember for a TH2004R Trans. Your 83 will probably need the Frame extension added, these are available from Summit, Jegs, Iceman etc..
DriveShaft will need to be shortened and Yoke changed, TH400 has 32-spline Yoke. A shorter Yoke may be available to avoid shortening the DS.
Trans Swap Info>> http://www.maliburacing.com/transmission_swaps.html